A Splatbook is a sourcebook for a particular role-playing game that is not needed for play, but is devoted to a particular facet, character class, or fictional faction, providing additional background details and rules options. For example, a "swords and sorcery" fantasy game might offer splatbooks for each of the races in the setting: humans, dwarves, elves, and others.

The term originally rose to describe the sourcebooks published by White Wolf Game Studio for its World of Darkness games.^[1] Many of these books were titled using similar patterns: clanbooks in Vampire: The Masquerade, tribebooks for Werewolf: The Apocalypse, traditionbooks for Mage: The Ascension, and so forth. In newsgroups, these were called *books (the asterisk on a computer keyboard being used as a wildcard character). Since the asterisk is also known as a "splat", this gave rise to the term "splatbook".^[2]

This term was subsequently used retrospectively for Advanced Dungeons & Dragons books such as The Complete Book of Dwarves and Complete Arcane,^[3] or the numerous codices for Warhammer Fantasy Battle and Warhammer 40,000. By extension, the term "splat" is used for the character class described in a splatbook.

In his 2023 book Monsters, Aliens, and Holes in the Ground, RPG historian Stu Horvath agreed that the term was used first for White Wolf's games in the 1990s, but called Cults of Prax, a 1979 sourcebook about religious cults created for the fantasy RPG RuneQuest, "the first splatbook ... the first book that fits, retroactively, the established definition."^[2] Game historian Shannon Appelcline also agreed, calling Cults of Prax "an early example" of a splatbook.^[1]