EX-5.1

We are acting as special counsel to Packaging Corporation of America, a Delaware corporation (the “Company”), in connection with the preparation of the Registration Statement on Form S-3ASR (such Registration Statement, as amended or supplemented, is hereinafter referred to as the “Registration Statement”) filed with the Securities and Exchange Commission (the “Commission”) on or about June 21, 2012 under the Securities Act of 1933, as amended (the “Securities Act”), by the Company. The Registration Statement relates to the issuance and sale from time to time, pursuant to Rule 415 of the General Rules and Regulations promulgated under the Securities Act, of an unspecified amount of debt securities (“Debt Securities”) in one or more offerings from time to time on a delayed or continuous basis.

You have advised us that the Debt Securities will be issued under the indenture, dated as of July 21, 2003, which was filed as an exhibit to the Company’s Quarterly Report on Form 10-Q filed with the Commission on August 13, 2003, between the Company and U.S. Bank National Association, as trustee (the “Trustee”), as may be supplemented by one or more additional supplemental indentures or officers’ certificates (collectively, the “Indenture”).

In connection with the registration of the Debt Securities, we have examined originals, or copies certified or otherwise identified to our satisfaction, of such documents, corporate records and other instruments as we have deemed necessary for the purposes of this opinion, including: (i) the organizational documents of the Company, (ii) minutes and records of the corporate proceedings of the Company and (iii) the Registration Statement and the exhibits thereto.

For purposes of this opinion, we have assumed the authenticity of all documents submitted to us as originals, the conformity to the originals of all documents submitted to us as copies and the authenticity of the originals of all documents submitted to us as copies. We have also assumed the legal capacity of all natural persons, the genuineness of the signatures of persons signing all documents in connection with which this opinion is rendered, the authority of such persons signing on behalf of the parties thereto other than the Company and the due

authorization, execution and delivery of all documents by the parties thereto other than the Company. We have not independently established or verified any facts relevant to the opinion expressed herein, but have relied upon statements and representations of the officers and other representatives of the Company.

(i) the Registration Statement will have become effective and comply with all applicable laws;

(ii) the Registration Statement will be effective and will comply with all applicable laws at the time the Debt Securities are offered or issued as contemplated by the Registration Statement;

(iii) a prospectus supplement or term sheet (“Prospectus Supplement”) will have been prepared and filed with the Commission describing the Debt Securities offered thereby and will comply with all applicable laws;

(iv) all Debt Securities will be issued and sold in compliance with applicable federal and state securities laws and in the manner stated in the Registration Statement and the appropriate Prospectus Supplement;

(v) the Debt Securities will be issued and sold in the form and containing the terms set forth in the Registration Statement, the appropriate Prospectus Supplement and the Indenture;

(vi) the Debt Securities offered, as well as the terms of the Indenture, as executed and delivered, do not violate any law applicable to the Company or result in a default under or breach of any agreement or instrument binding upon the Company;

(vii) the Company will have obtained any legally required consents, approvals, authorizations and other orders of the Commission and any other regulatory authorities necessary to issue and sell the Debt Securities being offered and to execute and deliver the Indenture;

(viii) the Debt Securities offered, as well as the terms of the Indenture, as executed and delivered, comply with all requirements and restrictions, if any, applicable to the Company, whether imposed by any court or governmental or regulatory body having jurisdiction over the Company;

(ix) a definitive underwriting agreement (“Underwriting Agreement”) with respect to any Debt Securities offered or issued will have been duly authorized and validly executed and delivered by the Company and the other parties thereto; and

(x) the Indenture and the Trustee will have been qualified under the Trust Indenture Act of 1939, as amended.

Based upon and subject to the foregoing qualifications, assumptions and limitations and the further limitations set forth below, we are of the opinion that:

When, as and if (a) the terms of any particular series of Debt Securities have been duly established in accordance with the Indenture and applicable law, (b) the appropriate corporate or organizational action has been taken to authorize the form, terms, execution and delivery of such Debt Securities (and any required amendment or supplement to the Indenture), and (c) the Debt Securities have been duly executed and delivered by duly authorized officers against payment therefor in accordance with such authorization, the applicable Indenture, the applicable Underwriting Agreement and applicable law and authenticated by the Trustee, such Debt Securities (including any Debt Securities duly executed and delivered upon the exchange or conversion of Debt Securities that are exchangeable or convertible into another series of Debt Securities) will be validly issued and will constitute binding obligations of the Company enforceable against the Company in accordance with their terms.

Our opinion expressed above is subject to the qualifications that we express no opinion as to the applicability of, compliance with, or effect of (i) any bankruptcy, insolvency, reorganization, fraudulent transfer, fraudulent conveyance, moratorium or other similar law affecting the enforcement of creditors’ rights generally, (ii) general principles of equity (regardless of whether enforcement is considered in a proceeding in equity or at law), (iii) public policy considerations which may limit the rights of parties to obtain certain remedies, and (iv) any laws except the laws of the State of New York and the State of Delaware.

We express no opinion with respect to the enforceability of (i) consents to, or restrictions upon, judicial relief or jurisdiction or venue; (ii) waivers of rights or defenses with respect to stay, extension or usury laws; (iii) advance waivers of claims, defenses, rights granted by law, or notice, opportunity for hearing, evidentiary requirements, statutes of limitation, trial by jury or at law, or other procedural rights; (iv) waivers of broadly or vaguely stated rights; (v) provisions for exclusivity, election or cumulation of rights or remedies; (vi) provisions authorizing or validating conclusive or discretionary determinations; (vii) grants of setoff rights; (viii) provisions for the

payment of attorneys’ fees where such payment is contrary to law or public policy; (ix) proxies, powers and trusts; (x) restrictions upon non-written modifications and waivers; (xi) provisions prohibiting, restricting, or requiring consent to assignment or transfer of any right or property; (xii) any provision to the extent it requires any party to indemnify any other person against loss in obtaining the currency due following a court judgment in another currency; and (xiii) provisions for liquidated damages, default interest, late charges, monetary penalties, make-whole premiums or other economic remedies to the extent such provisions are deemed to constitute a penalty. In addition, we express no opinion with respect to (i) whether acceleration of the Debt Securities may affect the collectibility of that portion of the stated principal amount thereof that might be determined to constitute unearned interest thereon, (ii) compliance with laws relating to permissible rates of interest or (iii) the creation, validity, perfection or priority of any security interest or lien.

To the extent that the obligations of the Company under the Indenture may be dependent on such matters, we assume for purposes of this opinion that the Trustee is duly organized, validly existing and in good standing under the laws of its jurisdiction of organization; that the Trustee is duly qualified to engage in the activities contemplated by the Indenture; that the Indenture has been duly authorized, executed and delivered by the Trustee and constitutes the legally valid and binding obligations of the Trustee, enforceable against the Trustee in accordance with its terms; that the Trustee is in compliance, generally and with respect to acting as trustee under the Indenture, with all applicable laws and regulations; and that the Trustee has the requisite organizational and legal power and authority to perform its obligations under the Indenture.

We hereby consent to the filing of this opinion with the Commission as Exhibit 5.1 to the Registration Statement. We also consent to the reference to our firm under the heading “Legal Matters” in the Registration Statement. In giving this consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission.

We do not find it necessary for the purposes of this opinion, and accordingly we do not purport to cover herein, the application of the securities or “Blue Sky” laws of the various states to the sale of the Debt Securities.

This opinion is limited to the specific issues addressed herein, and no opinion may be inferred or implied beyond that expressly stated herein. We assume no obligation to revise or supplement this opinion in response to changes in law or future events or other circumstances.

This opinion is furnished to you in connection with the filing of the Registration Statement, and is not to be used, circulated, quoted or otherwise relied upon for any other purpose.

Sincerely,

/s/ KIRKLAND & ELLIS LLP

KIRKLAND & ELLIS LLP