In the United States, a federal crime or federal offense is an act that is made illegal by U.S. federal legislation. In the United States, criminal law and prosecution happen at both the federal and the state levels; thus a “federal crime” is one that is prosecuted under federal criminal law, and not under a state's criminal law, under which most of the crimes committed in the United States are prosecuted.

This includes many acts that, if they did not occur on U.S. federal property or on Indian reservations or were not specifically penalized, would otherwise not be crimes or fall under state or local law. Some crimes are listed in Title 18 of the United States Code, but others fall under other titles; for instance, tax evasion and possession of weapons banned by the National Firearms Act are criminalized in Title 26 of the United States Code.

Numerous federal agencies have been granted powers to investigate federal offenses to include, but not limited to, the ATF, DEA, FBI, ICE, IRS, and the Secret Service.

Mail fraud which crosses state lines or involves the (national) United States Postal Service is a federal offense.
Other federal crimes include Aircraft hijacking, kidnapping, bank robbery, tax evasion, counterfeiting, Art theft from a museum, damaging or destroying public mailboxes, immigration offenses, and since 1965, assassinating the President or Vice President, although these were not made federal crimes until after President John F. Kennedy's assassination.

In drug-related federal offenses mandatory minimums can be enforced. A mandatory minimum is a federally regulated minimum sentence for offenses of certain drugs.

Prosecution guidelines are established by the United States Attorney in each federal judicial district and by laws that Congress has already established.