Desertion, in law, the act of abandoning a person, an obligation, or a station. In military law, desertion is absence from a unit or place of duty without authority. In the u.s. Army and u.s. Air Force, a person who abandons his or her unit or station is first considered to be absent without leave (AWOL); the u.s. Navy and u.s. Marine Corps use the term unauthorized absence (UA). If the person is still missing after 30 days, he or she is charged with desertion. In practice, many who have technically deserted are charged with a lesser offense.

In time of war, desertion is punishable by death. This sentence has been carried out by the American armed forces only once since the Civil War, during World War II.

In maritime law, desertion is abandonment of a ship by a sailor whose term of service has not yet expired.

In civil law, desertion is usually the act of abandoning one's spouse without intent to return. Desertion can also be (1) misconduct so serious that it would be expected to cause one's spouse to leave; or (2) failure to care for the needs of one's ailing spouse.