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 canceled after it failed to gain funding.

In 2000, during the development of Daikatana, Romero listed Ultima V: Warriors of Destiny, Super Mario Bros. 3, Age of Empires, Duke Nukem 3D and Chrono Trigger as his favorite games of all time, with Chrono Trigger topping the list In 2017, Romero listed World of Warcraft and Minecraft as his favorite games of all time.

Romero's favorite programming language as of 2017 is Lua. Romero says he has hyperthymesia. Romero is an atheist.He also claimed that everyone involved at working on the original Doom was an atheist (although game designer Sandy Petersen is a Mormon).On December 19, 2023, Romero acquired Irish citizenship, after living there for about 8 years.

Romero was referenced in the 2020 video game Doom Eternal as King Ormero.