497

State Street Bank and Trust Company

Fund Administration Legal Division

4 Copley Place, 5th floor

Boston, MA 02116

October 19, 2012

VIA EDGAR

U.S. Securities and Exchange Commission

100 F Street, N.E.

Washington, D.C. 20549

Attention:         Office of Filings, Information & Consumer Service

Re: State Street Institutional Investment Trust (the “Trust”)

File Nos.: 811-09819 and 333-30810

Dear Sir or Madam:

The Supplement is being filed to submit exhibits containing interactive data format risk/return summary information that mirrors the risk/return summary information in the Supplement dated October 1, 2012 to the Prospectuses dated April 30, 2012 for the State Street Institutional Liquid Reserves Fund, State Street Institutional Tax Free Money Market Fund, State Street Institutional U.S. Government Money Market Fund, State Street Institutional Treasury Money Market Fund, and State Street Institutional Treasury Plus Money Market Fund, as filed electronically via EDGAR with the Securities and Exchange Commission on October 1, 2012 (Accession # 0001193125-12-411436).

If you have any questions, please contact me at (617) 662-3967.

Very truly yours,

/s/ Mark E. Tuttle                    

Mark E. Tuttle

Vice President and Counsel

XML

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSIXX)

STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (SSTXX)

STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVMXX)

STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TRIXX)

STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPIXX)

INSTITUTIONAL CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, and 14:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

The following language is hereby added to footnote 2 on the table titled, “Annual Fund Operating Expenses” on pages 10 and 17:

The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSVXX)

STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (TFVXX)

STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVVXX)

STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TRVXX)

STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPVXX)

INVESTMENT CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, and 15:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

The following language is hereby added to footnote 2 on the table titled, “Annual Fund Operating Expenses” on pages 11 and 18:

The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (LRSXX)

STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (TASXX)

STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVSXX)

STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TYSXX)

STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPSXX)

SERVICE CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, 10, 13 and 16:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSLXX)

CLASS M SHARES

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on page 3:

⁽³⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Prospectus Date

rr_ProspectusDate

Apr. 30, 2012

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSIXX)

STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (SSTXX)

STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVMXX)

STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TRIXX)

STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPIXX)

INSTITUTIONAL CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, and 14:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

The following language is hereby added to footnote 2 on the table titled, “Annual Fund Operating Expenses” on pages 10 and 17:

The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSVXX)

STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (TFVXX)

STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVVXX)

STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TRVXX)

STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPVXX)

INVESTMENT CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, and 15:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

The following language is hereby added to footnote 2 on the table titled, “Annual Fund Operating Expenses” on pages 11 and 18:

The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (LRSXX)

STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (TASXX)

STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVSXX)

STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TYSXX)

STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPSXX)

SERVICE CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, 10, 13 and 16:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSLXX)

CLASS M SHARES

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on page 3:

⁽³⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Institutional Class | State Street Institutional Liquid Reserves Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSIXX)

INSTITUTIONAL CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, and 14:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Institutional Class | State Street Institutional Tax Free Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (SSTXX)

INSTITUTIONAL CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, and 14:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Institutional Class | State Street Institutional U.S. Government Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVMXX)

INSTITUTIONAL CLASS

The following language is hereby added to footnote 2 on the table titled, “Annual Fund Operating Expenses” on pages 10 and 17:

The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Institutional Class | State Street Institutional Treasury Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TRIXX)

INSTITUTIONAL CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, and 14:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Institutional Class | State Street Institutional Treasury Plus Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPIXX)

INSTITUTIONAL CLASS

The following language is hereby added to footnote 2 on the table titled, “Annual Fund Operating Expenses” on pages 10 and 17:

The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Investment Class | State Street Institutional Liquid Reserves Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSVXX)

INVESTMENT CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, and 15:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Investment Class | State Street Institutional Tax Free Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (TFVXX)

INVESTMENT CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, and 15:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Investment Class | State Street Institutional U.S. Government Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVVXX)

INVESTMENT CLASS

The following language is hereby added to footnote 2 on the table titled, “Annual Fund Operating Expenses” on pages 11 and 18:

The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Investment Class | State Street Institutional Treasury Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TRVXX)

INVESTMENT CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, and 15:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Investment Class | State Street Institutional Treasury Plus Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPVXX)

INVESTMENT CLASS

The following language is hereby added to footnote 2 on the table titled, “Annual Fund Operating Expenses” on pages 11 and 18:

The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Service Class | State Street Institutional Liquid Reserves Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (LRSXX)

SERVICE CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, 10, 13 and 16:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Service Class | State Street Institutional Tax Free Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (TASXX)

SERVICE CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, 10, 13 and 16:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Service Class | State Street Institutional U.S. Government Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVSXX)

SERVICE CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, 10, 13 and 16:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Service Class | State Street Institutional Treasury Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TYSXX)

SERVICE CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, 10, 13 and 16:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Service Class | State Street Institutional Treasury Plus Money Market Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPSXX)

SERVICE CLASS

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on pages 3, 7, 10, 13 and 16:

⁽²⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

Class M Shares | State Street Institutional Liquid Reserves Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Supplement Dated October 1, 2012

to the

Prospectus Dated April 30, 2012

STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSLXX)

CLASS M SHARES

The following footnote is hereby inserted after “Total Annual Fund Operating Expenses” on the table titled, “Annual Fund Operating Expenses” on page 3:

⁽³⁾  The Fund’s investment adviser, SSgA Funds Management, Inc. (the “Adviser” or “SSgA FM”), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the “Voluntary Reduction”) which may vary from time to time in the Adviser’s sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation.

XML

Document and Entity Information	12 Months Ended
	Apr. 30, 2012
Risk/Return:
Document Type	Other
Document Period End Date	Dec. 31, 2011
Registrant Name	STATE STREET INSTITUTIONAL INVESTMENT TRUST
Central Index Key	0001107414
Amendment Flag	false
Document Creation Date	Oct. 01, 2012
Document Effective Date	Oct. 01, 2012
Prospectus Date	Apr. 30, 2012

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	STATE STREET INSTITUTIONAL INVESTMENT TRUST
Prospectus Date	rr_ProspectusDate	Apr. 30, 2012
Document Creation Date	dei_DocumentCreationDate	Oct. 01, 2012