Qui tam is an abbreviation from the Latin "qui tam pro domino rege quam pro sic ipso in hoc parte sequitur" meaning "he who as well for the king as for himself sues in this matter."

Qui tam is the technical legal term for the mechanism in the federal False Claims Act that allows persons and entities with evidence of fraud against federal programs or contracts to sue the wrongdoer on behalf of the Government. A qui tam action is one brought under the False Claims Act by a private plaintiff on behalf of the Federal Government (rather than by the Government itself). The Government has the right to intervene and join the action, or the Government may decline intervention, in which case the private plaintiff may proceed on his own. The qui tam provisions in the False Claims Act include strong incentives both to report fraud against the Government and to participate in the resulting litigation.

Qui tam enforcement dates back to the Middle Ages, and was a common mechanism for protecting the public fisc in Anglo-Saxon jurisprudence. America's first Continental Congress borrowed from this tradition and passed several qui tam statutes.

The Federal False Claims Act, 31 U.S.C. §§ 3729-3733—sometimes referred to as "the whistleblower law"—was originally enacted in 1863 at the urging of President Abraham Lincoln, whose Union army was being routed by the rebels despite the rebels' inferior size and materials. At the time, many suppliers were defrauding the Union by, for example, selling it army crates filled with sawdust instead of muskets, and selling it the same cavalry horses two and three times. The Government needed an incentive for private individuals to help combat fraud, and thus the False Claims Act was born.

Despite nearly a century of successful fraud suppression, the qui tam provisions were amended in 1943, in such a way as to eliminate much of the incentive for private individuals to bring suit. Indeed, the amendments abolished the guaranteed 50% share of the proceeds that the whistleblower was awarded under the 1863 act, and added procedural hurdles which prevented many cases from going forward. As a result, fraud against the Government proliferated.

In 1986, President Ronald Reagan signed into law new amendments to the False Claims Act which restored the effectiveness of the Act. These amendments rejuvenated the Act, and have helped return billions of dollars to the treasury since 1986. In addition, a strong False Claims Act undoubtedly deters much fraud before it occurs.

Under the Act, successful relators are entitled to at least 15% and up to 30% of the funds they help recover for the Government. In addition, whistleblowers are protected from retaliation for activity relating to the qui tam action. Generally, in order for the most common FCA suit to be successful, a relator must prove by the preponderance of the evidence (more likely than not) that the defendant knowingly submitted a false claim for payment or approval to the Government.