Unknown Worlds Entertainment is an American video game developer based in San Francisco. The studio is best known for the Natural Selection and Subnautica series. In October 2021, the studio was acquired by South Korean video game developer Krafton.

Unknown Worlds was formed in May 2001 by Charlie Cleveland and began life as a group of developers responsible for the development of the high-profile free mod for Half-Life, Natural Selection. The success of Natural Selection convinced Cleveland to start work on a commercial sequel to the game: Natural Selection 2. Soon after, Cleveland founded Unknown Worlds Entertainment as a commercial computer games studio.

Although the casual games market is not the intended direction of Unknown Worlds, Zen of Sudoku, a casual puzzle computer game based on the popular logic puzzle Sudoku, was created in November 2006 in order to generate revenue towards funding the development of Natural Selection 2. Charlie Cleveland, one of the developers of Unknown Worlds Entertainment, cited casual games development as a "last option" for funding this sequel, having been unwilling to sacrifice control of the company to external investors.

In October 2006, Max McGuire became the studio's co-founder, having previously worked at Iron Lore Entertainment as Lead Engine Programmer. McGuire became the Technical Director of UWE and development of Natural Selection 2 began in earnest. Max and Charlie then attracted a group of angel investors including Richard Kain, Matthew Le Merle, Ira Rothken and Colin Wiel to back the company after a meeting at GDC in San Francisco. A year later, Unknown Worlds released Decoda as a commercial debugger for the Lua programming language. This application was created to aid with development of Natural Selection 2, whose game code was largely being written in Lua.

Later on in development of Natural Selection 2, the studio announced it had changed engine from the Source engine to their own proprietary engine developed in-house. After consulting their fanbase on a possible name for their new engine, it was finally named the Evolution engine. Later, it transpired that the name Evolution was already taken and Spark was chosen as the name for the engine.

In June 2008, Cory Strader was hired as art director. Strader had been a previous key member of the development team for Natural Selection. In May 2009, Unknown Worlds began taking pre-orders for standard and special edition versions of Natural Selection 2. Natural Selection 2 was released on 31 October 2012.

Perfect World acquired a 40% stake in Unknown Worlds Entertainment in August 2011 for CN¥10,803,330. It raised this to a controlling interest in February 2013 by acquiring a further 20% for CN¥5,098,956. During its 2019 fiscal year, Perfect World sold the majority of its shares in Unknown Worlds to the Founders. It sold the remainder in January 2021 to the founders at $50 million valuation, few months before the acquisition by Krafton.

In October 2021, South Korean video game holding company and publisher Krafton, known for overseeing titles such as PlayerUnknown's Battlegrounds and TERA through its subsidiaries, announced that it would acquire Unknown Worlds. Krafton stated that Unknown Worlds would continue to operate independently.