The Sega Mark III (セガマークIII) is a video game console created by Sega. It is the successor to Sega's earlier SG-1000 II (itself a redesigned SG-1000), and was released in October 1985 in Japan, before being distributed in South Korea and Taiwan later in the decade. No models of the Mark III were released outside of Asia, but the console did form the basis of the Sega Master System which was distributed internationally.

The Mark III stands as Sega's second attempt at capturing a share of the Japanese video game market in the face of the Family Computer (Famicom) created by Nintendo, sporting significant upgrades over its predecessors. The technology was used as a foundation for its successor, the Sega Mega Drive.