Ex parte Milligan, 71 U.S. 2 (1866), is a landmark decision of the U.S. Supreme Court that ruled that the use of military tribunals to try civilians when civil courts are operating is unconstitutional. In this particular case, the Court was unwilling to give former President Abraham Lincoln's administration the power of military commission jurisdiction, part of the administration's controversial plan to deal with Union dissenters during the American Civil War. Justice David Davis, who delivered the majority opinion, stated that "martial rule can never exist when the courts are open" and confined martial law to areas of "military operations, where war really prevails", and when it was a necessity to provide a substitute for a civil authority that had been overthrown. Chief Justice Salmon P. Chase and three associate justices filed a separate opinion concurring with the majority in the judgment, but asserting that Congress had the power to authorize a military commission, although it had not done so in Milligan's case.

David Davis was an American politician and jurist who was a U.S. senator from Illinois and associate justice of the United States Supreme Court. He also served as Abraham Lincoln's campaign manager at the 1860 Republican National Convention, engineering Lincoln's successful nomination for president by that party.