Phantasy Star (ファンタシースター, Fantashī Sutā) is a series of console role-playing video games and other supplementary media created by Sega. The series debuted in 1987 on the Master System with Phantasy Star, and continues into the present with Phantasy Star Online 2 and other extensions of the Phantasy Star Online sub-series. Each of the games in the series features a science fantasy setting featuring a cross-genre combination of magic and technology.

The first four games in the series are set in or related to the fictional planetary system of Algol, and are single-player turn-based role-playing games. Later games in the series are chiefly action role-playing games, and some contain massively multiplayer online game elements. There are currently four distinct Phantasy Star subseries, each set in their own universe. Cameos or nods to other subseries may appear, and there are concepts and terminology shared across them, similarly to Suikoden (Lagrange Point), Breath of Fire, Dragon Quest and Final Fantasy.