Altered Beast is a 1988 beat 'em up arcade video game developed and manufactured by Sega. The game is set in Ancient Greece and follows a player character chosen by Zeus to rescue his daughter Athena from the demonic ruler of the underworld, Neff. Through the use of power-ups, the player character can assume the form of different magical beasts (wolf, dragon, bear, tiger, and golden wolf). It was ported to several home video game consoles and home computers. It was the pack-in game for the Sega Mega Drive when that system launched in 1988.

The game was developed by Makoto Uchida, who developed the game as his first project as a lead developer. Uchida and his team used the System 16 arcade system board for its graphical capabilities with sprites. Altered Beast was ported numerous times in addition to its Genesis conversion, including for the Master System by Sega and to several computer systems and video game consoles by various third parties.

Altered Beast's arcade release and its various ports have all received mixed reviews, mainly targeting the game's gameplay and graphics. The game has been re-released several times for various consoles and as part of video game compilations, and two sequels to the game have been developed.