The Master System is an 8-bit third-generation home video game console manufactured and developed by Sega. It was originally a remodeled export version of the Sega Mark III, the third iteration of the SG-1000 series of consoles, which was released in Japan in 1985 and featured enhanced graphical capabilities over its predecessors. The Master System launched in North America in 1986, followed by Europe in 1987, and then in Brazil and Korea in 1989. A Japanese version of the Master System was also launched in 1987, which features a few enhancements over the export models (and by proxy the original Mark III): a built-in FM audio chip, a rapid-fire switch, and a dedicated port for the 3D glasses. The Master System II, a cheaper model, was released in 1990 in North America, Australasia and Europe.

The original Master System models use both cartridges and a credit card-sized format known as Sega Cards. Accessories for the consoles include a light gun and 3D glasses that work with a range of specially designed games. The later Master System II redesign removed the card slot, turning it into a strictly cartridge-only system, and is incompatible with the 3D glasses.

The Master System was released in competition with the Nintendo Entertainment System (NES). Its library is smaller and with fewer well-reviewed games than the NES, due in part to Nintendo licensing policies requiring platform exclusivity. Though the Master System had newer, improved hardware, it failed to overturn Nintendo's significant market share advantage in Japan and North America. However, it attained significantly greater success in other markets, including Europe, Brazil, South Korea and Australia.

The Sega Master System is estimated to have sold between 10 and 13 million units worldwide. In addition, Tectoy has sold 8 million licensed Master System variants in Brazil. Retrospective criticism has recognized its role in the development of the Sega Genesis, and a number of well-received games, particularly in PAL (including PAL-M) regions, but is critical of its limited library in the NTSC regions, which were dominated by the NES.