Orbiter is a space flight simulator program developed to simulate spaceflight using realistic Newtonian physics. The simulator was released on 27 November 2000; the latest edition, labeled "Orbiter 2016", was released on 30 August 2016, the first new version of the simulator since 2010. On 27 July 2021, Dr Schweiger announced to the Orbiter Community that Orbiter is being published under open source MIT license.

Orbiter was developed by Martin Schweiger, a senior research fellow in the computer science department at University College London, who felt that space flight simulators at the time were lacking in realistic physics-based flight models, and decided to write a simulator that made learning physics concepts enjoyable. It has been used as a teaching aid in classrooms, and a community of add-on developers have created a multitude of add-ons to allow users to fly assorted real and fictional spacecraft and add new planets or planetary systems.