Serious Sam II (or Serious Sam 2) is a science fiction first-person shooter video game released for the PC and Xbox and the sequel to the 2002 computer game Serious Sam: The Second Encounter, making it the third game in the Serious Sam series. It was designed and developed by Croteam and was released on October 11, 2005. The game was initially published by 2K Games, a Take-Two Interactive subsidiary. The game was later made available on Steam on January 31, 2012. While the game was originally released only for Windows and the Xbox, a Linux version of the game client and the game's content editor, Serious Editor 2, was later released and is in beta.

In the single-player campaign, the player assumes the role of hero Sam "Serious" Stone in his adventures against the forces of the extraterrestrial overlord, "Mental", who seeks to destroy humanity. Taking place after the events of Serious Sam: The Second Encounter, Sam travels through various worlds collecting parts of a medallion in an effort to defeat Mental. He is guided by the Sirian Great Council and receives sporadic aid from the natives of the worlds he visits. The multiplayer mode includes online co-op and deathmatch, the latter having been introduced in a patch. A 4.5/5.0 was awarded to Serious Sam by Computer Gaming World, though overall the game received moderate praise from the media, earning an average of 75% on Game Rankings.

Croteam simultaneously developed Serious Engine 2, the successor to their previous game engine, Serious Engine, for use in the game, and the engine is capable of many features of other advanced game engines of the time including high dynamic range rendering and light bloom. The engine supports integration with both Xfire and GameSpy Arcade for multiplayer match finding. Serious Sam II is the only game so far to use the proprietary engine, though the Serious Engine II is available for licensing.