A chess variant is any board game related to, derived from, or inspired by orthodox chess. Chess variants can differ from standard chess in many different ways, including variant board sizes, non-standard fairy pieces, alternative starting positions, and additional rules. There are thousands of known chess variants (see list of chess variants). The Classified Encyclopedia of Chess Variants catalogues around two thousand, with the preface noting that—since creating a chess variant is relatively easy—many were considered insufficiently notable for inclusion. While most chess variants are not widely played, some have reached modest levels of popularity, with the most successful being Chess960. Several well known modern chess masters like Capablanca, Fischer, and Yasser Seirewan developed their own chess variants.

"Classical", "orthodox" or "Western" chess itself is one of a family of games which have related origins and could be considered part of the same board game family. Modern chess itself developed from the Indian chaturanga via the Middle-eastern shatranj. During the Middle Ages, there were numerous popular chess variants, like Great chess and Courier chess. Modern chess variants are mostly all variations on standard Western chess. Though not technically variants of modern chess, there are also many regional games which are part of the same family, such as shogi (Japan), sittuyin (Burma), makruk (Thailand), hiashatar (Mongolia), and xiangqi (China). These games have their own history of variants (see shogi variants and xiangqi variants).

Many modern chess variants are designed to be played with the standard chess board and chessmen (such as chess960 and crazyhouse), but others require special equipment such as unique boards and chessmen to represent the fairy pieces. Most variants have a similar public-domain status as their parent game, but some have been made into commercial proprietary games. Just as in traditional chess, chess variants can be played over the board, by correspondence, or by computer. Some internet chess servers facilitate the play of some variants in addition to orthodox chess.

In the context of chess problems, chess variants are also called heterodox chess or fairy chess. Fairy chess variants tend to be created for problem composition rather than actual play.

A Chess variant or a heterodox chess game is any game which closely resembles classic chess with one or more differences. These differences can include alternative pieces, boards, rules or even number of players. The term variety of Chess was in use by the late 19th century. British chess historian H. J. R. Murray discusses different variants in his A History of Chess (1913), indicating that he considers the term "chess" to refer to a family of games that includes not just modern classical chess but all the games which I traced back to the Indian chaturanga, and all the freak modifications that have been attempted from time to time.

There are thousands of chess variants. According to Cazaux & Knowlton in their A World of Chess that "the world of chess variants is endless. Pritchard offers more than 1600 samples in his Encyclopedia of Chess Variants, and the Chess Variants Pages on the Internet have several thousand entries."

While some regional variants have historical origins comparable to or even older than modern orthodox chess, the majority of chess variants are modern attempts by individuals or small groups to create new variations on orthodox chess. Variants can themselves be developed into further sub-variants, for example Capablanca chess led to numerous adaptions and Horde chess is a variation upon Dunsany's Chess.

Some chess variants were created to address a perceived issue with the traditional game of chess. For example, chess960 (also known as Fischer random chess), which randomizes the starting positions of the pieces on the first and eighth ranks, was introduced by grandmaster Bobby Fischer to combat what he perceived to be the detrimental dominance of opening preparation in chess.