Document

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 the following securities: (i) common stock of Commvault, par value $0.01 per share (the “Common Stock”); (ii) preferred stock of Commvault, par value $0.01 per share (the “Preferred Stock”); (iii) depositary shares of Commvault representing fractional interests of Preferred Stock (the “Depositary Shares”); (iv) one or more series of debt securities of Commvault (the “Debt Securities”); (v) warrants to purchase Debt Securities, Common Stock or Preferred Stock (the “Warrants”); (vi) stock purchase contracts, pursuant to which the holder will purchase from Commvault a specified number of shares of Common Stock at a future date or dates (the “Stock Purchase Contracts”); and (vii) stock purchase units, consisting of a Stock Purchase Contract and a security (of Commvault or another issuer) securing the holder’s obligation to purchase the Common Stock under the Stock Purchase Contracts (“Stock Purchase Units” and, collectively with the Common Stock, the Preferred Stock, the Depositary Shares, the Debt Securities, the Warrants and the Stock Purchase Contracts, the “Securities”), in one or more offerings from time to time on a delayed or continuous basis.

You have advised us that (i) the Debt Securities will be issued under the Indenture, dated as of October 28, 2026, by and between Commvault and U.S. Bank Trust Company, National Association, as trustee (the “Trustee”) (including any amendments or supplements thereto, the “Indenture”), (ii) the Depositary Shares will be issued pursuant to one or more deposit agreements (including a form of depositary receipt evidencing the Depositary Shares) (each, a “Deposit Agreement”), by and between Commvault and a depositary (each, a “Depositary”), in a form that will be filed as an exhibit to the Registration Statement or incorporated by reference therein, (iii) the Warrants will be issued pursuant to one or more warrant agreements (including a form of certificate evidencing the Warrants) (each, a “Warrant Agreement”), by and between Commvault and a bank or trust company named

therein as the warrant agent (the “Warrant Agent”), in a form that will be filed as an exhibit to the Registration Statement or incorporated by reference therein, (iv) the Stock Purchase Contracts will be issued pursuant to one or more stock purchase contracts (including a form of Stock Purchase Contract), by and among Commvault and the other parties named therein, in a form that will be filed as an exhibit to the Registration Statement or incorporated by reference therein and (v) the Stock Purchase Units will be issued under one or more unit agreements (including a form of Stock Purchase Unit) (each, a “Unit Agreement”), by and among Commvault and the other parties named therein, in a form that will be filed as an exhibit to the Registration Statement or incorporated by reference therein. The Indenture and each Deposit Agreement, Warrant Agreement, Stock Purchase Contract and Unit Agreement shall be referred to herein as a “Governing Document.” The Trustee and each Depositary, Warrant Agent or other counterparty to a Governing Document shall be referred to herein as a “Governing Document Counterparty.”

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 and the due authorization, execution and delivery of all documents by the parties thereto (other than, with respect to due authorization, Commvault). We have not independently established or verified any facts relevant to the opinions expressed herein, but have relied upon statements and representations of the officers and other representatives of Commvault and others as to factual matters.

(v)at the time of the issuance, sale and delivery of each Security, (1) the authorization of such Security by Commvault will not have been modified or rescinded, and there will not have occurred any change in law affecting the validity, legally binding character or enforceability of such Security and (2) the issuance, sale and delivery of such Security, the terms of such Security, the terms of the applicable Governing Document and the compliance by Commvault with the terms of such Security and the terms of the applicable Governing Document will not violate any applicable law, any agreement or instrument then binding upon Commvault, or cause default thereunder, or any restriction imposed by any court or governmental body having jurisdiction over Commvault;

1.When, as and if (a) the shares of Common Stock to be offered and sold by Commvault have been duly authorized by appropriate corporate action of Commvault and (b) such shares of Common Stock have been issued, sold and delivered against payment therefor (which shall be in an amount at least equal to the par value of the shares being issued and sold) in accordance with such authorization, the applicable Purchase Agreement and applicable law (including any shares of Common Stock delivered upon exercise, conversion or exchange of any Securities that are exercisable, convertible into or exchangeable for shares of Common Stock pursuant to the terms thereof, assuming that such Securities have been duly authorized and established in accordance with applicable law, appropriate corporate action has been taken to authorize the form, terms, issuance and delivery of such Securities and such Securities have been issued, sold and delivered against payment therefor by duly authorized officers in accordance with such authorization, the applicable Purchase Agreement, any applicable Governing Document and applicable law), such shares of Common Stock will be validly issued, fully paid and non-assessable.

of such Preferred Stock have been issued, sold and delivered against payment therefor (which shall be in an amount at least equal to the par value of the shares being issued and sold) in accordance with such authorization, the applicable Purchase Agreement and applicable law (including any shares of Preferred Stock delivered upon exercise, conversion or exchange of any Securities that are exercisable, convertible into or exchangeable for shares of Preferred Stock pursuant to the terms thereof, assuming that such Securities have been duly authorized and established in accordance with applicable law, appropriate corporate action has been taken to authorize the form, terms, issuance and delivery of such Securities and such Securities have been issued, sold and delivered against payment therefor by duly authorized officers in accordance with such authorization, the applicable Purchase Agreement, any applicable Governing Document and applicable law), such shares of Preferred Stock will be validly issued, fully paid and non-assessable.

3.When, as and if (a) any Debt Securities have been duly authorized and duly established in accordance with the Indenture and applicable law, (b) appropriate corporate action has been taken to authorize the form, terms, execution and delivery of such Debt Securities and the Indenture and the Indenture has been duly executed and delivered in accordance with such authorization and (c) the applicable Debt Securities have been duly executed, attested, issued and delivered by duly authorized officers against payment in accordance with such authorization, the Indenture, the applicable Purchase Agreement and applicable law, and duly authenticated by the Trustee in accordance with the Indenture, such Debt Securities (including any Debt Securities delivered upon exercise, conversion or exchange of any Securities that are exercisable, convertible into or exchangeable for Debt Securities pursuant to the terms thereof, assuming that such Securities have been duly authorized and established in accordance with applicable law, appropriate corporate action has been taken to authorize the form, terms, issuance and delivery of such Securities and such Securities have been issued, sold and delivered against payment therefor in accordance with such authorization, the applicable Purchase Agreement, any applicable Governing Document and applicable law) will be validly issued and will constitute binding obligations of Commvault enforceable against Commvault in accordance with their terms.

4.When, as and if (a) any shares of Preferred Stock underlying the Depositary Shares have been duly authorized and established and, if appropriate, reserved for issuance and sale in accordance with the applicable Deposit Agreement, Commvault’s organizational documents and applicable law, (b) appropriate corporate action has been taken by Commvault to authorize the form, terms, execution and delivery of the Deposit Agreement and the Depositary Shares, and the Deposit Agreement has been duly executed and delivered by the Depositary and Commvault in accordance with such authorization, (c) the shares of Preferred Stock represented by the Depositary Shares have been duly issued and delivered to the Depositary under the applicable Deposit Agreement and (d) the depositary receipts evidencing the Depositary Shares have been duly executed, attested, issued and delivered by the Depositary and/or duly authorized officers of Commvault against payment in accordance with the applicable authorization, applicable Purchase Agreement, applicable Deposit Agreement and applicable law, such Depositary Shares will be validly issued and will constitute binding obligations of Commvault enforceable against Commvault in accordance with their terms.

5.When, as and if (a) any Warrants have been duly authorized and duly established in accordance with applicable law, (b) appropriate corporate action has been taken by Commvault to authorize the form, terms, execution and delivery of the Warrant Agreement and the Warrants and the Warrant Agreement has been duly executed and delivered by the Warrant Agent and Commvault in accordance with such authorization, (c) Warrants with such terms have been duly executed, attested, issued and delivered by the Warrant Agent and/or duly authorized officers of Commvault against payment in accordance with such authorization, the applicable Purchase Agreement, applicable Warrant Agreement

and applicable law and (d) the Securities issuable upon exercise of such Warrants have been duly authorized and reserved for issuance by all necessary corporate action and, in the case of Warrants to purchase Debt Securities, when the terms of such Debt Securities have been duly authorized and duly established in accordance with the terms of the Indenture, applicable law and the appropriate corporate action and, in the case of Warrants to purchase Common Stock or Preferred Stock, that the exercise price of such Warrants consists of legal consideration at least equal to the par value of such securities, such Warrants will be validly issued and will constitute binding obligations of Commvault enforceable against Commvault in accordance with their terms.

6.When, as and if (a) any Stock Purchase Contracts have been duly authorized and duly established in accordance with applicable law, (b) appropriate corporate action has been taken by Commvault to authorize the form, terms, execution and delivery of such Stock Purchase Contracts, (c) Stock Purchase Contracts with such terms have been duly executed, attested, issued and delivered by any applicable Governing Document Counterparty and/or duly authorized officers of Commvault against payment in accordance with such authorization, the applicable Purchase Agreement, applicable Stock Purchase Contract and applicable law, and (d) the Common Stock issuable upon exercise of such Stock Purchase Contracts has been duly authorized and reserved for issuance by all necessary corporate action, such Stock Purchase Contracts will be validly issued and will constitute binding obligations of Commvault enforceable against Commvault in accordance with their terms.

7.When, as and if (a) any Stock Purchase Units have been duly authorized and duly established in accordance with applicable law, (b) appropriate corporate action has been taken by Commvault to authorize the form, terms, execution and delivery of the Stock Purchase Units (including a form of certificate evidencing the Stock Purchase Units) and the related Unit Agreement, (c) the related Unit Agreement has been duly executed and delivered by any applicable Governing Document Counterparty and Commvault in accordance with such authorization, (d) the Stock Purchase Units with such terms have been duly executed, attested, issued and delivered by any applicable Governing Document Counterparty and/or duly authorized officers of Commvault against payment in accordance with such authorization, the applicable Purchase Agreement, applicable Unit Agreement and applicable law and (e) the Common Stock and any other Securities issuable upon exercise of such Stock Purchase Units have been duly authorized and reserved for issuance by all necessary corporate action and, in the case of any Debt Securities issuable thereunder, the terms of such Debt Securities have been duly authorized and duly established in accordance with the terms of the Indenture, applicable law and the appropriate corporate action, such Stock Purchase Units will be validly issued and will constitute binding obligations of Commvault enforceable against Commvault in accordance with their terms.

Our opinions expressed above that any particular contract constitutes a valid and binding obligation or is enforceable in accordance with its terms (each, an “enforceability opinion”) are subject to: (i) the effect of bankruptcy, insolvency, fraudulent conveyance and other similar laws and judicially developed doctrines in this area, such as substantive consolidation and equitable subordination; (ii) the effect of general principles of equity; and (iii) other commonly recognized statutory and judicial constraints on enforceability, including statutes of limitations. “General principles of equity” include but are not limited to: (A) principles limiting the availability of specific performance and injunctive relief; (B) principles which limit the availability of a remedy under certain circumstances where another remedy has been elected; (C) principles requiring reasonableness, good faith and fair dealing in the performance and enforcement of an agreement by the party seeking enforcement; (D) principles which may permit a party to cure a material failure to perform its obligations; and

(E) principles affording equitable defenses such as waiver, laches and estoppel. It is possible that terms in a particular contract covered by our enforceability opinion may not prove enforceable for reasons other than those explicitly cited in this opinion should an actual enforcement action be brought, but (subject to all the exceptions, qualifications, exclusions and other limitations contained in this opinion) such unenforceability would not in our opinion prevent the party entitled to enforce that contract from realizing the principal benefits purported to be provided to that party by the terms in that contract which are covered by our enforceability opinion.

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; (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; and (xiv) any rights to contribution or indemnification which may be violative of public policy underlying any law, rule or regulation (including federal or state securities law, rule or regulation) or the enforceability of any so-called fraudulent conveyance or fraudulent transfer “savings clause” (and any similar provision in any other document or agreement) to the extent such provisions purport to limit the amount of the obligations of any party or the right to contribution of any other party with respect to such obligations. In addition, we express no opinion with respect to (i) whether acceleration of the Debt Securities may affect the collectability 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.

Counterparty is duly qualified to engage in the activities contemplated by the applicable Governing Document; that the applicable Governing Document has been duly authorized, executed and delivered by the applicable Governing Document Counterparty and constitutes the legally valid and binding obligations of such Governing Document Counterparty, enforceable against such Governing Document Counterparty in accordance with its terms; that the applicable Governing Document Counterparty is in compliance, generally and with respect to acting as Trustee, Warrant Agent, Depositary or other Governing Document Counterparty, as the case may be, under the applicable Governing Document, with all applicable laws and regulations; and that the applicable Governing Document Counterparty has the requisite organizational and legal power and authority to perform its obligations under the applicable Governing Document.

This opinion is limited to the specific issues addressed herein, and no opinion may be inferred or implied beyond that expressly stated herein. The Securities may be issued from time to time on a delayed or continuous basis, and this opinion is limited to the laws, including the rules and regulations, as in effect on the date hereof, which laws are subject to change with possible retroactive effect. This opinion speaks only as of the date hereof and we assume no obligation to revise or supplement this opinion after the date hereof.



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Exhibit 5.1



Commvault Systems, Inc. October 28, 2026 Page 2



Commvault Systems, Inc. October 28, 2026 Page 3



Commvault Systems, Inc. October 28, 2026 Page 4