A Game Master (often abbreviated as GM) is a participant in a multiplayer roleplaying game who acts as organizer, arbitrator, and referee in rules situations. It is the Game Master's responsibility is to weave the players' stories together, control the non-player aspects of the game, and create environments in which the players can interact. The basic roles of Game Masters - rules help, moderation, and storytelling - are the same in almost all role-playing games.

The Game Master prepares the game session for the players and the characters they play (known as player characters or PCs). The GM describes the events and decides on the outcomes of players' decisions. The Game Master also keeps track of non-player characters (NPCs) and random encounters, as well as of the general state of the game world.

The game session (or "adventure") can be metaphorically described as a collectively written and improvised play, in which the players are the lead actors, and the GM provides the stage, the scenery, the basic plot on which the improvisational script is built, as well as all the bit parts and supporting characters.

A good Game Master draws the players into the adventure, making it enjoyable for everyone. Good Game Masters have quick minds, sharp wits, and rich imaginations. Game Masters must also maintain game balance: hideously overpowered monsters or players are no fun. It was noted back in 1997 that those who favor their left-brain such as skilled code writers usually do not make it in the ethereal Game Master world of storytelling and verse.