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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.

In Robert Axelrod's book The Evolution of Cooperation, Grim Trigger is called FRIEDMAN.

view</span>	Topics in game theory
Definitions	Normal form game · Extensive form game · Cooperative game · Information set · Preference
Equilibrium concepts	Nash equilibrium · Subgame perfection · Bayes-Nash · Trembling hand · Correlated equilibrium · Sequential equilibrium · Quasi-perfect equilibrium · Evolutionarily stable strategy
Strategies	Dominant strategies · Mixed strategy · Grim trigger · Tit for Tat
Classes of games	Symmetric game · Perfect information · Dynamic game · Repeated game · Signaling game · Cheap talk · Zero-sum game · Mechanism design
Games	Prisoner's dilemma · Coordination game · Chicken · Battle of the sexes · Stag hunt · Matching pennies · Ultimatum game · Minority game · Rock, Paper, Scissors · Pirate game · Dictator game
Theorems	Minimax theorem · Purification theorems · Folk theorem · Revelation principle · Arrow's Theorem
Related topics	Mathematics · Economics · Behavioral economics · Evolutionary game theory · Population genetics · Behavioral ecology · Adaptive dynamics · List of game theorists