A peace treaty is an agreement between two or more hostile parties, usually countries or governments, which formally ends a state of war between the parties. It is different from an armistice, which is an agreement to stop hostilities; a surrender, in which an army agrees to give up arms; or a ceasefire or truce, in which the parties may agree to temporarily or permanently stop fighting.

The need for a peace treaty in modern diplomacy arises from the fact that even when a war is actually over and fighting has ceased, the legal state of war is not automatically terminated upon the end of actual fighting and the belligerent parties are still legally defined as enemies. This is evident from the definition of a "state of war" as "a legal state created and ended by official declaration regardless of actual armed hostilities and usually characterized by operation of the rules of war". As a result, even when hostilities are over, a peace treaty is required for the former belligerents in order to reach agreement on all issues involved in transition to legal state of peace. The art of negotiating a peace treaty in the modern era has been referred to by legal scholar Christine Bell as the lex pacificatoria, with a peace treaty potentially contributing to the legal framework governing the post conflict period, or jus post bellum.

Since 1950, the rate at which interstate wars end with a formal peace treaty has substantially declined.