Valve Corporation 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 franchises Half-Life, Counter-Strike, Portal, Day of Defeat, Team Fortress, Left 4 Dead, and Dota.

Valve was founded in 1996 by 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; with its realism, scripted sequences and seamless narrative, it had a lasting influence on the FPS genre. Harrington left in 2000. In 2003, Valve launched Steam, followed by Half-Life 2 in 2004. With advanced physics systems and an increased focus on story and characters, Half-Life 2 received critical acclaim and sold 12 million copies by 2011. In 2006, Valve released the episodic sequel Half-Life 2: Episode One, followed in 2007 by Episode Two, the multiplayer game Team Fortress 2 and the puzzle game Portal. Portal 2 was released in 2011. In 2013, Valve released the multiplayer online battle arena game Dota 2.

Valve released fewer games in the 2010s 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 50 to 70% of the market for downloaded PC games in 2011 and generated an estimated $3.4 billion in 2017.