Advancements in computer performance and chess engine development have culminated in virtually all users of online chess sites having the means to access engine play far superior to that of even the world's strongest human players. Some users employ engine assistance while in play, which is considered cheating in most cases. To combat this, the most prominent online chess platforms, Chess.com and Lichess, devote significant resources to detecting and handling cheaters, and cheaters employ methods of evading detection, such as cheating only occasionally, in turn.

It is often impossible to determine with absolute certainty whether a player is cheating. Chess sites typically scrutinize multiple games in their analyses of prospective cheaters to limit false positives, but they still rarely occur. Ways of dealing with detected cheaters include permanent but appealable bans, used by Chess.com, and secretly matchmaking cheaters with each other, used by Lichess.

Titled players, even grandmasters, have been caught cheating online. Cheaters have also been found in online tournaments with cash prizes, such as Titled Tuesday, despite these tournaments implementing rigorous and intrusive anti-cheating measures. Accusations of online cheating have been levied between titled players, including by former World Champion Vladimir Kramnik.

Both Chess.com and Lichess forbid players from receiving any outside assistance for their play in rated games (with the exception of correspondence games, in which opening repertoires may be referred to). Despite this, some players refer to engines while in play, giving them an unfair advantage over their opponents. In 2022, it was estimated that the world's strongest chess engine, Stockfish, which is free and open source, could be expected to beat the world's strongest human player, Magnus Carlsen, in 98% of games, drawing him in the remaining 2%. For ordinary chess players, prone to inaccuracies and blunders, defeating engine assisted play is only possible if and when cheaters make mistakes, if and when they decide not to use engine assistance.

Roughly 0.6% of Chess.com accounts have been closed for cheating. Chess.com claims it analyzed 2.5 million games in 2023, and that it banned 1 million accounts for cheating that same year.. Since International Master Danny Rensch has claimed that the site only conducts bans it is "willing to go to court" over , the actual number of cheaters can be significantly higher.

Lichess claims that it handled 91,000 reports for cheating in 2022 and 93,000 in 2023, and that it flagged 61,000 and 72,000 accounts for cheating in those two years, respectively; it's unclear how many of the flagged accounts were also reported.

Chess sites employ numerous tracking and analytical methods in their attempts to accurately detect cheaters. Lichess maintains an open source machine learning tool for cheating detection, but does not disclose whether it relies on other, closed source tools. While most cheaters are obvious enough to be detected automatically, more difficult cases require human analysis. Speaking for Chess.com, International Master Danny Rensch has claimed that the site only conducts bans it is "willing to go to court" over.

Chess.com and Lichess differ in how they handle accounts they determine to be cheating. Chess.com publicly issues permanent bans, visible as a crossed red circle icon next to the names of banned users. In addition, the site refunds the rating points of players who have recently lost games to banned accounts. In contrast, Lichess tends to secretly place detected cheaters in a separate playing pool, which prevents cheaters from knowing when they have been caught. Chess.com and Lichess both allow detected cheaters to appeal their ban statuses. Of roughly 39,000 appeals processed by Chess.com in 2023, about 0.3% were granted.