Third Party Complaint Law and Legal Definition

A third-party complaint is a complaint filed against a third party by a defendant or plaintiff alleging that the third party is liable for all or part of a claim or counterclaim in dispute between the original parties.

For example, the Federal Rules of Civil Procedure provide that at any time after commencement of the action a defending party, as a third-party plaintiff, may cause a summons and complaint to be served upon a person not a party to the action who is or may be liable to the third-party plaintiff for all or part of the plaintiff's claim against the third-party plaintiff.