Exhibit 5.1

300 North LaSalle Street

Chicago, Illinois 60654

June 26, 2012

Packaging Corporation of America

1955 West Field Court

Lake Forest, Illinois 60045

Ladies and Gentlemen:

We have acted as special counsel to Packaging Corporation of America, a Delaware corporation (the “Company”), in connection with the issuance and sale by the Company of $400,000,000 in aggregate principal amount of 3.900% Senior Notes due 2022 (the “Notes”), in connection with a Registration Statement (File No. 333-182251) on Form S-3ASR filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), on June 21, 2012. Such Registration Statement, as amended or supplemented, is hereinafter referred to as the “Registration Statement.” The Notes were issued pursuant to the Indenture, dated as of July 21, 2003 (the “Indenture”), by and between the Company and U.S. Bank National Association, as trustee (the “Trustee”), as supplemented by the Officers’ Certificate establishing the terms of the Notes, dated June 26, 2012 (the “Officers’ Certificate”).

In connection with the issuance of the Notes, 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, (iii) the Registration Statement and the exhibits thereto, (iv) the Indenture, (v) the Officers’ Certificate and (vi) copies of the Notes.

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

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June 26, 2012

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expressed herein, but have relied upon statements and representations of the officers and other representatives of the Company.

Based upon and subject to the foregoing qualifications, assumptions and limitations and the further limitations set forth below, we are of the opinion that the Notes have been validly issued and 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 Delaware General Corporation Law.

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 Notes 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,

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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 Company’s Current Report on Form 8-K dated June 26, 2012. 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 Notes.

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 in accordance with the requirements of Item 601(b)(5)(i) of Regulation S-K promulgated under the Securities Act, and is not to be used, circulated, quoted or otherwise relied upon for any other purpose.

Sincerely,

/s/ KIRKLAND & ELLIS LLP

KIRKLAND & ELLIS LLP