A gamemaster (GM), also known as a game master, game manager, game moderator, referee, storyteller, or master of ceremonies, is a person who acts as a facilitator, organizer, officiant regarding rules, arbitrator, and moderator for a multiplayer role-playing game. The act performed by a gamemaster is sometimes referred to as "gamemastering" or simply "GM-ing."

The role of a GM in a traditional tabletop role-playing game (TTRPG) is to weave together the other participants' player characters (PCs) stories, control the non-player characters (NPCs), describe or create environments in which the PCs can interact, and solve any player disputes. This basic role is the same in almost all traditional TTRPGs, with minor differences specific to differing rule sets. However, in some indie role-playing games, the GM role significantly differs from the traditional pattern. For example, in Powered by the Apocalypse systems, the other players assist the GM in creating both the NPCs and the details of the campaign setting.

The role of a gamemaster in an online game is to enforce the game's rules and provide general customer service.

Gaming systems have their own names for the role of the GM. For example, in Dungeons & Dragons, they are called Dungeon Masters, in the World of Darkness games, they are called storytellers, and in Powered by the Apocalypse games they are called a variety of names, such as MCs (master of ceremonies).

GMs are typically hobbyists; however, they are sometimes paid employees or entertainers for hire. This is more common for online games. Paid GMing was very uncommon for TTRPGs before the 2020s.

In a role-playing game context, the term gamemaster was first used by Dave Arneson while developing his game Blackmoor in 1971, although the first usage in print may have been Chivalry & Sorcery.

Each gaming system has its own name for the role of the gamemaster, such as "judge," "narrator," "referee," "director," or "storyteller," and these terms not only describe the role of the GM in general but also help define how the game is intended to be run. For example, the most famous of such terms, the "Dungeon Master" (or "DM") in Dungeons & Dragons, highlights the game's focus on dungeon crawling. The Storyteller System used in White Wolf Game Studio's storytelling games calls its GM the "storyteller," while the rules- and setting-focused Marvel Super Heroes role-playing game calls its GM the "judge." The cartoon inspired role-playing game Toon calls its GM the "animator." Some games apply flavorful names to the GM to fit the genre or setting, such as the Keeper of Arcane Lore (in the occult-themed Call of Cthulhu), the Hollyhock God (Nobilis, in which the hollyhock represents vanity), the Groundskeeper (in the spooky Bluebeard's Bride), the Mall Rat (in Visigoths vs. Mall Goths), or the Gaymaster (in LGBTQ-centered Thirsty Sword Lesbians).

The term gamemaster and the role associated with it have been used in the postal gaming hobby since the 1980s. In typical play-by-mail games, players control armies or civilizations and mail their chosen actions to the GM. The GM then mails the updated game state to all players on a regular basis. Usage in a wargaming context includes Guidon Games 1973 ruleset, Ironclad.