Ultra vires is a Latin phrase meaning literally "beyond the powers", although its standard legal translation and substitute is "beyond power". If an act requires legal authority and it is done with such authority, it is characterised in law as intra vires (literally "within the powers"; standard legal translation and substitute, "within power"). If it is done without such authority, it is ultra vires. Acts that are intra vires may equivalently be termed "valid" and those that are ultra vires "invalid".

In corporate law, ultra vires describes acts attempted by a corporation that are beyond the scope of powers granted by the corporation's objects clause, articles of incorporation or in a clause in its Bylaws, in the laws authorizing a corporation's formation, or similar founding documents. Acts attempted by a corporation that are beyond the scope of its charter are void or voidable.

Several modern developments relating to corporate formation have limited the probability that ultra vires acts will occur. Except in the case of non-profit corporations (including municipal corporations), this legal doctrine is obsolescent; within recent years, almost all business corporations are chartered to allow them to transact any lawful business. The Model Business Corporation Act of the United States states that: "The validity of corporate action may not be challenged on the ground that the corporation lacks or lacked power to act." The doctrine still has some life among non-profit corporations or state-created corporate bodies established for a specific public purpose, like universities or charities.