Grim Trigger is a trigger strategy in game theory for a repeated game, such as an iterated prisoner's dilemma. Initially, a player using Grim Trigger will cooperate, but as soon as the opponent defects (thus satisfying the trigger condition), the player using Grim Trigger will defect for the remainder of the iterated game. Since a single defect by the opponent triggers defection forever, Grim Trigger is the most strictly unforgiving of strategies in an iterated game.

Note that Tit for Tat, the commonly accepted optimal iterated game strategy, is also a trigger strategy, except that unlike Grim Trigger, it only "remembers" one move previous when determining whether to cooperate or defect.