Federal common law is a term used in the United States to describe common law that is developed by the federal courts, instead of by the courts of the various states. Although the United States Supreme Court has effectively barred the creation of federal common law in areas traditionally under the authority of state courts, there are several areas where federal common law continues to govern.

Up until 1938, the federal courts followed the doctrine set forth in the case of Swift v. Tyson, 41 U.S. 1 (1842). In that case, the United States Supreme Court had held that federal courts hearing cases brought under their diversity jurisdiction (allowing them to hear cases between parties from different states) had to apply the statutory law of the states, but not the common law developed by state courts. Instead, the Supreme Court permitted the federal courts to make their own common law based on general principles of law.

In 1938, however, the Supreme Court decided Erie Railroad v. Tompkins, 304 US 64 (1938), which over-ruled Swift v. Tyson, deciding instead that federal courts exercising diversity jurisdiction had to use all of the same substantive laws as the courts of the states in which they were located.

Nevertheless, there remain several areas of law where federal common law is allowed to continue, particularly where the Constitution of the United States gives the U.S. Congress power to make laws in an area, such as admiralty law, antitrust, bankruptcy law, and civil rights. Congress often lays down broad mandates with vague standards, which are then left to the courts to interpret, and these interpretations eventually give rise to complex understandings of the original intent of Congress, informed by the courts' understanding of what is just and reasonable.