In common law, a writ of qui tam is a writ through which private individuals who assist a prosecution can receive for themselves all or part of the damages or financial penalties recovered by the government as a result of the prosecution. Its name is an abbreviation of the Latin phrase qui tam pro domino rege quam pro se ipso in hac parte sequitur, meaning "[he] who sues in this matter for the king as well as for himself."

The writ fell into disuse in England and Wales following the Common Informers Act 1951 but remains current in the United States under the False Claims Act, 31 U.S.C. § 3729 et seq., which allows a private individual, or "whistleblower" (or relator), with knowledge of past or present fraud committed against the federal government to bring suit on its behalf. There are also qui tam provisions in 18 U.S.C. § 962 regarding arming vessels against friendly nations; 25 U.S.C. § 201 regarding violating Indian protection laws; 46 U.S.C. § 80103 regarding the removal of undersea treasure from the Florida coast to foreign nations; and 35 U.S.C. § 292 regarding false marking. In February 2011, the qui tam provision regarding false marking was held to be unconstitutional by a U.S. District Court, and in September of that year, the enactment of the Leahy–Smith America Invents Act effectively removed qui tam remedies from § 292.