State Street Institutional Investment Trust

State Street Institutional Investment Trust

SUPPLEMENT DATED AUGUST 8, 2018

TO THE PROSPECTUS AND THE STATEMENT OF ADDITIONAL INFORMATION

EACH DATED APRIL 30, 2018


STATE STREET TARGET RETIREMENT 2015 FUND Class A (SSBBX) Class I (SSBFX) Class K (SSBHX)	STATE STREET TARGET RETIREMENT 2020 FUND Class A (SSBJX) Class I (SSBNX) Class K (SSBOX)	STATE STREET TARGET RETIREMENT 2025 FUND Class A (SSBPX) Class I (SSBRX) Class K (SSBSX)

STATE STREET TARGET RETIREMENT 2030 FUND Class A (SSBUX) Class I (SSBWX) Class K (SSBYX)	STATE STREET TARGET RETIREMENT 2035 FUND Class A (SSBZX) Class I (SSCJX) Class K (SSCKX)	STATE STREET TARGET RETIREMENT 2040 FUND Class A (SSCLX) Class I (SSCNX) Class K (SSCQX)

STATE STREET TARGET RETIREMENT 2045 FUND Class A (SSCUX) Class I (SSDDX) Class K (SSDEX)	STATE STREET TARGET RETIREMENT 2050 FUND Class A (SSDFX) Class I (SSDJX) Class K (SSDLX)	STATE STREET TARGET RETIREMENT 2055 FUND Class A (SSDMX) Class I (SSDOX) Class K (SSDQX)

STATE STREET TARGET RETIREMENT 2060 FUND Class A (SSDTX) Class I (SSDWX) Class K (SSDYX)		STATE STREET TARGET RETIREMENT FUND Class A (SSFLX) Class I (SSFNX) Class K (SSFOX)
(each a “Fund” and together the “Funds”)

Effective as of September 4, 2018, the contractual operating expense waiver and reimbursement agreement for each Fund is amended to lower the cap on total annual fund operating expenses, exclusive of certain fees and expenses, from 0.13% to 0.09% of each Fund’s average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2019 except with approval of the Fund’s Board of Trustees. Accordingly, effective as of September 4, 2018, the Prospectus is revised as follows:

STATE STREET TARGET RETIREMENT 2015 FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 1 of the Prospectus and “Expenses” on page 2 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		0.42	%		0.42	%		0.22	%
Acquired Fund Fees and Expenses		0.15	%		0.15	%		0.15	%
Total Annual Fund Operating Expenses		0.87	%		0.62	%		0.42	%

Less Fee Waivers and/or Expense Reimbursements³		(0.33	)%		(0.33	)%		(0.33	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2019 (i) to waive up to the full amount of the advisory fee payable by the Fund, and/or (ii) to reimburse the Fund to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.09% of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2019 except with approval of the Fund’s Board of Trustees.

Example

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2019. Although your actual costs may be higher or lower, based on these assumptions your costs would be:


	1 year		3 years		5 years		10 years
Class A	$	503	$	684	$	880	$	1,445
Class I	$	30	$	165	$	313	$	743
Class K	$	9	$	101	$	202	$	498

STATE STREET TARGET RETIREMENT 2020 FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 11 of the Prospectus and “Expenses” on page 12 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		0.30	%		0.30	%		0.10	%
Acquired Fund Fees and Expenses		0.11	%		0.11	%		0.11	%
Total Annual Fund Operating Expenses		0.71	%		0.46	%		0.26	%

Less Fee Waivers and/or Expense Reimbursements³		(0.17	)%		(0.17	)%		(0.17	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Example


	1 year		3 years		5 years		10 years
Class A	$	503	$	651	$	811	$	1,278
Class I	$	30	$	130	$	241	$	563
Class K	$	9	$	66	$	129	$	314

STATE STREET TARGET RETIREMENT 2025 FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 21 of the Prospectus and “Expenses” on page 22 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		0.30	%		0.30	%		0.10	%
Acquired Fund Fees and Expenses		0.09	%		0.09	%		0.09	%
Total Annual Fund Operating Expenses		0.69	%		0.44	%		0.24	%

Less Fee Waivers and/or Expense Reimbursements³		(0.15	)%		(0.15	)%		(0.15	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Example


	1 year		3 years		5 years		10 years
Class A	$	503	$	646	$	803	$	1,256
Class I	$	30	$	126	$	231	$	540
Class K	$	9	$	62	$	120	$	291

STATE STREET TARGET RETIREMENT 2030 FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 31 of the Prospectus and “Expenses” on page 32 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		0.30	%		0.30	%		0.10	%
Acquired Fund Fees and Expenses		0.07	%		0.07	%		0.07	%
Total Annual Fund Operating Expenses		0.67	%		0.42	%		0.22	%

Less Fee Waivers and/or Expense Reimbursements³		(0.13	)%		(0.13	)%		(0.13	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Example


	1 year		3 years		5 years		10 years
Class A	$	503	$	642	$	794	$	1,235
Class I	$	30	$	122	$	222	$	517
Class K	$	9	$	58	$	111	$	267

STATE STREET TARGET RETIREMENT 2035 FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 41 of the Prospectus and “Expenses” on page 42 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		0.32	%		0.32	%		0.12	%
Acquired Fund Fees and Expenses		0.05	%		0.05	%		0.05	%
Total Annual Fund Operating Expenses		0.67	%		0.42	%		0.22	%

Less Fee Waivers and/or Expense Reimbursements³		(0.13	)%		(0.13	)%		(0.13	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Example


	1 year		3 years		5 years		10 years
Class A	$	503	$	642	$	794	$	1,235
Class I	$	30	$	122	$	222	$	517
Class K	$	9	$	58	$	111	$	267

STATE STREET TARGET RETIREMENT 2040 FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 51 of the Prospectus and “Expenses” on page 52 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		0.32	%		0.32	%		0.12	%
Acquired Fund Fees and Expenses		0.06	%		0.06	%		0.06	%
Total Annual Fund Operating Expenses		0.68	%		0.43	%		0.23	%

Less Fee Waivers and/or Expense Reimbursements³		(0.14	)%		(0.14	)%		(0.14	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Example


	1 year		3 years		5 years		10 years
Class A	$	503	$	644	$	798	$	1,246
Class I	$	30	$	124	$	227	$	529
Class K	$	9	$	60	$	115	$	279

STATE STREET TARGET RETIREMENT 2045 FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 61 of the Prospectus and “Expenses” on page 62 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		0.36	%		0.36	%		0.16	%
Acquired Fund Fees and Expenses		0.06	%		0.06	%		0.06	%
Total Annual Fund Operating Expenses		0.72	%		0.47	%		0.27	%

Less Fee Waivers and/or Expense Reimbursements³		(0.18	)%		(0.18	)%		(0.18	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Example


	1 year		3 years		5 years		10 years
Class A	$	503	$	653	$	816	$	1,288
Class I	$	30	$	133	$	245	$	574
Class K	$	9	$	69	$	134	$	325

STATE STREET TARGET RETIREMENT 2050 FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 71 of the Prospectus and “Expenses” on page 72 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		0.40	%		0.40	%		0.20	%
Acquired Fund Fees and Expenses		0.06	%		0.06	%		0.06	%
Total Annual Fund Operating Expenses		0.76	%		0.51	%		0.31	%

Less Fee Waivers and/or Expense Reimbursements³		(0.22	)%		(0.22	)%		(0.22	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Example


	1 year		3 years		5 years		10 years
Class A	$	503	$	661	$	833	$	1,330
Class I	$	30	$	141	$	263	$	619
Class K	$	9	$	77	$	152	$	372

STATE STREET TARGET RETIREMENT 2055 FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 81 of the Prospectus and “Expenses” on page 82 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		0.67	%		0.67	%		0.47	%
Acquired Fund Fees and Expenses		0.06	%		0.06	%		0.06	%
Total Annual Fund Operating Expenses		1.03	%		0.78	%		0.58	%

Less Fee Waivers and/or Expense Reimbursements³		(0.49	)%		(0.49	)%		(0.49	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Example


	1 year		3 years		5 years		10 years
Class A	$	503	$	716	$	948	$	1,610
Class I	$	30	$	200	$	385	$	920
Class K	$	9	$	136	$	275	$	679

STATE STREET TARGET RETIREMENT 2060 FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 91 of the Prospectus and “Expenses” on page 92 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		3.23	%		3.23	%		3.03	%
Acquired Fund Fees and Expenses		0.06	%		0.06	%		0.06	%
Total Annual Fund Operating Expenses		3.59	%		3.34	%		3.14	%

Less Fee Waivers and/or Expense Reimbursements³		(3.05	)%		(3.05	)%		(3.05	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Example


	1 year		3 years		5 years		10 years
Class A	$	503	$	1,229	$	1,976	$	3,938
Class I	$	30	$	741	$	1,476	$	3,422
Class K	$	9	$	680	$	1,377	$	3,234

STATE STREET TARGET RETIREMENT FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 101 of the Prospectus and “Expenses” on page 102 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)


	Class A			Class I			Class K
Management Fee		0.05	%		0.05	%		0.05	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses²		0.51	%		0.51	%		0.31	%
Acquired Fund Fees and Expenses		0.15	%		0.15	%		0.15	%
Total Annual Fund Operating Expenses		0.96	%		0.71	%		0.51	%

Less Fee Waivers and/or Expense Reimbursements³		(0.42	)%		(0.42	)%		(0.42	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.54	%		0.29	%		0.09	%

¹	A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²	Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Example


	1 year		3 years		5 years		10 years
Class A	$	503	$	702	$	918	$	1,538
Class I	$	30	$	185	$	354	$	843
Class K	$	9	$	121	$	243	$	600

The third paragraph of the sub-section entitled “Management and Organization – Investment Adviser” on page 124 of the Prospectus is deleted in its entirety and replaced with the following:

Total Annual Fund Operating Expense Waivers. SSGA FM, as the investment adviser to each Fund, is contractually obligated, through April 30, 2019, (i) to waive up to the full amount of the advisory fee payable by a Fund, and/or (ii) to reimburse a Fund for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.09% of average daily net assets on an annual basis. These waiver and/or reimbursement agreements may not be terminated prior to April 30, 2019 except with approval of the Funds’ Board of Trustees.

In addition, effective as of September 4, 2018, the Statement of Additional Information (the “SAI”) is revised as follows:

The sub-section entitled “Total Annual Fund Operating Expense Waivers” in the section entitled “Investment Advisory and Other Services – Investment Advisory Agreement” on pages 64 and 65 of the SAI is deleted in its entirety and replaced with the following:

Total Annual Fund Operating Expense Waivers. The Adviser has contractually agreed with the Trust, through April 30, 2019, (i) to waive up to the full amount of the advisory fee payable by certain Funds, and/or (ii) to reimburse a Fund for expenses to the extent that Total Annual Fund Operating Expenses (subject to certain exclusions as described in each Fund’s Prospectus) exceed the following percentage of average daily net assets on an annual basis with respect to the below-listed Funds:


Fund	Expense Limitation
Equity 500 Index Fund		0.01	%
Aggregate Bond Index Fund		0.04	%
Target Retirement 2015 Fund		0.09	%
Target Retirement 2020 Fund		0.09	%
Target Retirement 2025 Fund		0.09	%
Target Retirement 2030 Fund		0.09	%
Target Retirement 2035 Fund		0.09	%
Target Retirement 2040 Fund		0.09	%
Target Retirement 2045 Fund		0.09	%
Target Retirement 2050 Fund		0.09	%
Target Retirement 2055 Fund		0.09	%
Target Retirement 2060 Fund		0.09	%
Target Retirement Fund		0.09	%
Hedged International Developed Equity Index Fund		0.15	%
International Developed Equity Index Fund		0.09	%
Small/Mid Cap Equity Index Fund		0.03	%
Emerging Markets Equity Index Fund		0.12	%
Disciplined U.S. Equity Fund		0.65	%
Disciplined International Equity Fund		0.85	%
Disciplined Global Equity Fund		0.75	%
Global Value Spotlight Fund		0.70	%
European Value Spotlight Fund		0.70	%
Asia Pacific Value Spotlight Fund		0.70	%
U.S. Value Spotlight Fund		0.60	%
International Value Spotlight Fund		0.70	%

The Adviser has contractually agreed with the Trust, through April 30, 2020, (i) to waive up to the full amount of the advisory fee payable by the Global Equity ex-U.S. Index Fund, and/or (ii) to reimburse the Fund for expenses to the extent that Total Annual Fund Operating Expenses (subject to certain exclusions as described in the Fund’s Prospectus) exceed the following percentage of average daily net assets on an annual basis with respect to the Fund:


Fund	Expense Limitation
Global Equity ex-U.S. Index Fund		0.015	%

PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE

SUPSSIIT2018