Valve Corporation, also known as Valve Software, is an American video game developer, publisher, and digital distribution company headquartered in Bellevue, Washington. It is the developer of the software distribution platform Steam and the game franchises Half-Life, Counter-Strike, Portal, Day of Defeat, Team Fortress, Left 4 Dead and Dota.

Valve was founded in 1996 by the former Microsoft employees Gabe Newell and Mike Harrington. Their debut game, the first-person shooter (FPS) Half-Life (1998), was a critical and commercial success and had a lasting influence on the FPS genre. Harrington left in 2000. In 2003, Valve launched Steam, followed by Half-Life 2 (2004), the episodic sequels Half-Life 2: Episode One (2006) and Episode Two (2007), the multiplayer games Team Fortress 2 (2007) and Left 4 Dead (2008), the puzzle games Portal (2007) and Portal 2 (2011) and the multiplayer online battle arena game Dota 2 (2013).

In the 2010s, Valve released fewer games and experimented with hardware and virtual reality (VR). They entered the hardware market in 2015 with the Steam Machine, a line of gaming computers, which sold poorly, and released the HTC Vive and Valve Index VR headsets. They returned to the Half-Life series in 2020 with Half-Life: Alyx, their flagship VR game. In 2022, Valve released the Steam Deck, a portable gaming system.

Valve uses a flat structure, whereby employees decide what to work on themselves. They develop games through playtesting and iteration, describing game design as a kind of experimental psychology. By 2012, Valve employed around 250 people and was reportedly worth over US$3 billion. Most of Valve's revenue comes from Steam, which controlled over half of the digital PC games market in 2011 and generated an estimated $3.4 billion in 2017.