A legally binding set of federal rules adopted by the federal courts to govern issues that are not expressly addressed by the terms of a constitutional or statutory provision. The federal courts may develop federal common law to protect federal interests (for example, resolving conflicts among the states) or to apply if Congress has authorized the federal courts to establish substantive law.

For example, in the context of the Employee Retirement Income Security Act of 1974 (ERISA), the federal courts have developed federal common law to fill statutory gaps (including the applicable statutes of limitation under ERISA, rules governing coordination of benefits disputes, the make-whole doctrine, agency law, and waivers by a designated beneficiary) (see Firestone Tire & Rubber Co. v. Bruch, 489 U.S. 101, 110 (1989) (observing that Congress empowered the courts to "develop a federal common law of rights and obligations under ERISA-regulated plans"); Salyers v. Metro. Life Ins. Co., 871 F.3d 934, 940 (9th Cir. 2017) (collectin cases; adopting principles of agency law into federal common law in the context of an ERISA-governed life insurance dispute)). As another example, federal common law may need to be created if ERISA preempts a state law but does not otherwise address the issue that is preempted under state law (see Practice Note, ERISA Litigation: Preemption of State Laws (Overview): Federal Common Law and Preemption).