A court-martial or court martial (plural courts-martial or courts martial, as "martial" is a postpositive adjective) is a military court or a trial conducted in such a court. A court-martial is empowered to determine the guilt of members of the armed forces subject to military law, and, if the defendant is found guilty, to decide upon punishment. In addition, courts-martial may be used to try prisoners of war for war crimes. The Geneva Conventions require that POWs who are on trial for war crimes be subject to the same procedures as would be the holding military's own forces. Finally, courts-martial can be convened for other purposes, such as dealing with violations of martial law, and can involve civilian defendants.

Most navies have a standard court-martial which convenes whenever a ship is lost; this does not presume that the captain is suspected of wrongdoing, but merely that the circumstances surrounding the loss of the ship be made part of the official record. Most military forces maintain a judicial system that tries defendants for breaches of military discipline. Some countries like France have no courts-martial in times of peace and use civilian courts instead.