Alfonso John Romero (born October 28, 1967) is an American video game developer. He co-founded id Software and designed their early games, including Wolfenstein 3D (1992), Doom (1993), Doom II (1994), Hexen (1995) and Quake (1996). His designs and development tools, along with programming techniques developed by the id programmer John Carmack, popularized the first-person shooter (FPS) genre. Romero is also credited with coining the multiplayer term "deathmatch".

Following disputes with Carmack, Romero was fired from id in 1996. He co-founded a new studio, Ion Storm, and directed the FPS Daikatana (2000), which was a critical and commercial failure. Romero departed Ion Storm in 2001. In July 2001, he and another former id employee, Tom Hall, founded Monkeystone Games to develop games for mobile devices.

In 2003, Romero joined Midway Games as the project lead on Gauntlet: Seven Sorrows (2005), and left shortly before its release. He founded another company, Gazillion Entertainment, in 2005. In 2016, Romero and another former id employee, Adrian Carmack, announced a new FPS, Blackroom, but it was cancelled after failing to find a publisher.