Valve Corporation (also known as Valve Software or simply as Valve, and stylized as VALV^E) is an American video game development and digital distribution company based in Bellevue, Washington, United States. Founded in 1996 by former Microsoft employees Gabe Newell and Mike Harrington, Valve became famous from its critically acclaimed Half-Life series, (the first game released in November 1998). It is also well known for its social-distribution network Steam; and for developing the Source engine, which has been used in every Valve game since its introduction in 2004.

Valve was founded by long-time Microsoft employees Gabe Newell and Mike Harrington on August 24, 1996 as an L.L.C. based in Kirkland, Washington. After incorporation in April 2003, it moved from its original location to Bellevue, Washington, the same city in which their original publisher, Sierra On-Line, Inc., was based.

After the success of Half-Life, the team worked on mods, spin-offs, and sequels, including Half-Life 2. All current Valve games are built on its Source engine, which owes much of its success to mods and sequels. The company has developed six game series: Half-Life, Team Fortress, Portal, Counter-Strike, Left 4 Dead and Day of Defeat. Valve is noted for its support of its games' modding community: most prominently, Counter-Strike, Team Fortress, and Day of Defeat. Valve has branched out with this tradition to continue developing the Warcraft III-based mod Defense of the Ancients with Dota 2. Each of these games began as a third-party mod that Valve purchased and developed into a full game. They also distribute community mods on Steam.