The Sega Master System or SMS for short (1986 - 2000), is an 8-bit cartridge-based video game console that was manufactured by Sega. Its original Japanese incarnation was the SG-1000 Mark III. In the European market, this console launched Sega onto a competitive level comparable to Nintendo, due to its wider availability, but failed to put a dent in the North American and Japanese markets. The Master System was released as a direct competitor to the NES/Famicom. Despite its shakier performance in the major territories, it has enjoyed over a decade of life in secondary markets, especially Brazil.

The later Sega Game Gear is effectively a hand-held Master System, with a few enhancements.

The SG-1000 Mark III was released in Japan on October 20, 1985 to compete with the Famicom, following on from the SG-1000 Mark I and SG-1000 Mark II. The Mark III was built similarly to the Mark II, with the addition of improved video hardware and an increased amount of RAM. The system is backwards compatible with earlier SG-1000 titles. As well as the standard cartridge slot, it has a built-in slot for "Sega Cards", which are physically identical to the cards for the Sega SG-1000 "Card Catcher" add-on. The Mark III was redesigned as the Sega Master System for release in other markets. This was mainly a cosmetic revamp; the internals of the console remained virtually the same. The redesigned console was itself released in Japan in 1987, with the addition of a built-in Yamaha YM2413 FM sound chip, Rapid Fire Unit, and 3-D glasses adapter; these were sold separately for the Mark III. Sega Master System game cartridges released outside Japan had a different shape and pin configuration to the Japanese Master System/Mark III cartridges. This may be seen as a form of regional lockout. Typical of the era, game consoles had a mascot character. Sega's first mascot was Opa-opa from Fantasy Zone, as referenced in the manual for Zillion. Later on, especially in Western territories where Fantasy Zone was less popular, Alex Kidd emerged as a mascot. It is unclear if his mascot status was ever official, or if it were simply perceived because of the similarity to the Mario games that represented the competing Nintendo console. When Sonic the Hedgehog became the official Sega mascot in 1991, games were also produced for the Master System, but none of these were ever released in Japan for the system, the Game Gear being the platform who got these ports. Neither the Mark III nor the Japanese Sega Master System were commercially successful, due to strong competition from the Nintendo Famicom. The last commercial licensed release in Japan was Bomber Raid, released by Sega on February 4, 1989.

The system was redesigned and sold in the United States under the name Sega Master System in June 1986, less than a year after the Nintendo Entertainment System was released. The console sold for $200. The Master System was subsequently released in other locales and markets, including a second release in Japan in 1987 under the new Master System name. The Japanese Master System included a built-in 3-D glasses adapter, rapid fire, and a Yamaha YM2413 FM sound chip, all of which were separate accessories for the Mark III. Though the Master System was a more technically advanced piece of hardware than the NES, it did not attain the same level of popularity among consumers in the United States. Its lack of success in the U.S. has been attributed to various causes, among them the difference in game titles available for each platform and the slightly later release date of the Master System. The licensing agreement that Nintendo had with its third-party game developers had a profound impact. The agreement stated, in effect, that developers would exclusively produce games for the NES. The Master System sold 125,000 consoles in the first four months. In the same period, the NES would net 2,000,000. Nintendo had 90% of the North American market at the time. Hayao Nakayama, then CEO of Sega, decided not to use too much effort to market the console in the NES-dominated market. In 1988, the rights to the Master System in North America were sold to Tonka, but its popularity continued to decline. The move was considered a very bad one, since Tonka had never marketed a video game system and had no idea what to do with it. In 1990, Sega was having success with its Sega Genesis and as a result took back the rights from Tonka for the SMS. It designed the Sega Master System II, a low-cost Master System that lacked the reset button and card slot of the original. In an effort to counter Nintendo's Super Mario Brothers, the new system included Alex Kidd in Miracle World playable without any cartridges. Sega marketed the Master System II heavily; nevertheless, the unit sold poorly in North America. By 1992, the Master System's sales were virtually nonexistent in North America and sales in this market ceased. Sales were poor in Japan as well, due to the dominance of the main competitor from Nintendo, the Nintendo Famicom. The Master System left the Japanese market during 1989. The last commercial licensed release in North America was Sonic the Hedgehog, released by Sega on October 25, 1991.

In Europe, the Master System was very successful. Sega marketed the Master System in many countries, including several in which Nintendo did not sell its consoles. It had some success in Germany, where it was distributed by Ariolasoft beginning in winter 1987. In France, during the time the Sega Master System has been on sale, the console was distributed by the Virgin Group. In the United Kingdom, it was distributed by Mastertronic, who later merged with the Virgin Group. In Italy it was distributed by Giochi Preziosi and in his first years it overshadowed the Nintendo Entertainment System. The NES gained a good market share only later in his lifespan, with Sega Mega Drive already released. The Europeans had garnered lots of third party support for the SMS and as a result, it was able to outsell the NES in Europe, as opposed to what happened in Japan and North America where the console was discontinued much earlier. Nintendo was forced to get licensing for some popular SMS titles in that market, but the NES never reached the amount of console sales gathered by the Master System in the region, resulting on the 8-bit top-selling console in Europe. Like in North America Sega was able to bring the redesigned Sega Master System II into the shelves. This system included Alex Kidd in Miracle World, or later Sonic the Hedgehog, as a built in game. The Master System was supported until 1996 in Europe, the same time as its successor, the Mega Drive, which was also successful in Europe. Both were discontinued so that Sega could concentrate on the new Sega Saturn.

Sales of the SMS in Australia were exceptional as it was far more popular than the NES there; the SMS was able to gain greater market share there than that in North America due to Sega distributors Ozisoft having strong ties with retailers. In New Zealand it was largely successful as well - due to NES having a weak influence - and was supported until 1997.

Brazil was one of the SMS' most successful markets. It was marketed in that country by Tec Toy, Sega's Brazilian distributor. At least five versions were released between 1989 and 1995 and several games had been translated into Portuguese. The characters in the said games had been modified so that they appealed to Brazilian mainstream audiences (for example, Wonder Boy in Monster Land featured Mônica, the main character from a popular children's comic book in Brazil, created by Maurício de Sousa). Brazil also produced 100% national titles, like Sítio do Pica Pau Amarelo (based on Monteiro Lobato workmanship) and Castelo Ra-Tim-Bum (from the TV Cultura series). One of the more notable Master System consoles in Brazil was the compact wireless Master System Compact developed by Tec Toy. The console transmitted the A/V signal through RF, dispensing cable connections. It was produced from 1994 to 1997 and is still a target for console collectors. A similar version, called Master System Girl, was also released in an attempt to attract female consumers. The only difference in this version was a strong pink casing and pastel buttons. Later in its life in Brazil, Game Gear games had been ported to the Master System and several original Brazilian titles were made for the system. Tec Toy also produced a licensed version of the wildly popular fighting game Street Fighter II for the Master System. Despite the limitations of the console, the game turned out to be fairly well received. The console production was familiar to the Brazilians, which explains the success in that market. The last commercial licensed release in Brazil was Sonic Blast, released by Tec Toy on 1998. The Master System is still being produced in Brazil. The latest version is the "Master System III Collection". It uses the same design as the Master System II (Master System III in Brazil), but is white and comes in three versions: one with 74 games built-in, other with 105 games and another with 112 games. However, in Brazil, it is hard to find the 3-D Glasses, the Light Phaser and even cartridges, leaving most Brazilians with only built-in games. Overall, the SMS was mildly successful worldwide, but failed to capture the Japanese and North American markets. Sega learned from its mistakes and made the succeeding Sega Mega Drive/Genesis wildly popular in Europe, Brazil, and North America. The Sega Master System was rereleased in a smaller handheld form factor in late 2006. This small handheld device is powered by 3 AAA batteries, has a brighter active matrix screen, and contained 20 Game Gear and Sega Master System games. It was released under several brands including Coleco and PlayPal.