Portal is the original game of the Portal series. It is a single-player puzzle-based video game developed by Valve and released as part of The Orange Box on October 9, 2007 for Microsoft Windows and Xbox 360; the PlayStation 3 version released at the later date of December 11, 2007. Portal was also re-released as part of the launch of Mac OS X support for Steam on May 12, 2010; to celebrate it was given away for free until May 24, 2010 to Steam users on a promotional page on the Steam website. Further to this, Portal has been made available for free again for a limited duration as part of the Learn With Portals project.

Portal has been called one of the most innovative games of the decade. A hybrid of FPS style and a new genre of spatial brain teasers, Portal offers hours of totally unique gameplay. Set in the mysterious Aperture Science Laboratories, players must solve physical puzzles and challenges by opening portals, maneuvering objects, and moving themselves through space in ways that used to be impossible.