The Immortal Game was a chess game played in 1851 between Adolf Anderssen and Lionel Kieseritzky during the London 1851 chess tournament, an event in which both players participated. It was itself a casual game, however, not played as part of the tournament. Anderssen won the game by sacrificing all of his major pieces while developing a mating attack with his remaining minor pieces. Despite losing the game, Kieseritzky was impressed with Anderssen's performance. Kieseritzky published the game shortly afterwards in La Régence, a French chess journal which he helped to edit. Ernst Falkbeer published an analysis of the game in 1855, describing it for the first time with its sobriquet "immortal".

The Immortal Game is among the most famous chess games ever played. As a miniature game, it is frequently reproduced in chess literature to teach simple themes of gameplay. Although Kieseritzsky himself indicated that the game ended before checkmate, the Immortal Game is frequently reproduced with a brief continuation involving a queen sacrifice—a further loss of material—leading to checkmate. This continuation is commonly presented as part of the complete game, as if the final moves were actually played as part of the real historical game. Some authors also permute certain moves, deviating from Kieseritzky's report, although such permutations typically transpose to distinct lines of play that eventually return to the moves and positions reported by Kieseritzky.

Although both players made moves that are regarded as unsound by modern players, the game is appreciated as an example of the Romantic school of chess, a style of play that prized bold attacks and sacrifices over deep strategy. The game—especially its mating continuation—is also appreciated for its aesthetic value, as a plausible example of how a player with a significant material deficit but having an advantageous position can give mate. The continuation's mating position is a model mate, a strong form of pure mate (i.e. all of the attacker's remaining pieces contribute to the checkmate, while the mated king is prevented from moving to any other square for exactly one reason per square). In 1996, Bill Hartston called the game an achievement "perhaps unparalleled in chess literature".

Anderssen was one of the strongest players of his time, and many consider him to have been the world's strongest player after his victory in the London 1851 chess tournament. Kieseritzky lived in France much of his life, where he gave chess lessons and played games for five francs an hour at the Café de la Régence in Paris. His strength was shown most favourably when giving substantial odds to weak players; against masters, he was less convincing.

The Immortal Game was an informal one, played during a break in a formal tournament in London; the exact venue is uncertain. Kieseritzky was very impressed with Anderssen's performance; after the game was over, Kieseritzky telegraphed the moves of the game to his Parisian chess club. The French chess magazine La Régence published the game in July 1851. The Austrian Ernst Falkbeer nicknamed it "The Immortal Game" in 1855.

This game is acclaimed as an exemplar of the 19th-century Romantic style of chess, where rapid development and attack were considered the most effective way to win, many gambits and countergambits were offered (and not accepting them would be considered slightly ungentlemanly), and material was often held in contempt. These games, with their rapid attacks and counterattacks, are often entertaining to review, even if some of the moves are no longer considered optimal.

In this game, Anderssen won despite sacrificing a bishop (on move 11), both rooks (starting on move 18), and the queen (on move 22) to produce checkmate against Kieseritzky, who lost only three pawns. Anderssen later demonstrated the same kind of approach in the Evergreen Game.

Some published versions of the game have errors, as described in the annotations below.