The Sega Mark III was released in October 1985 as a successor to the SG-1000 II. Architecturally speaking, the Mark III is almost identical to the SG-1000, except that the Mark III has a heavily upgraded graphics processor (VDP) and more RAM, allowing for larger and prettier games. The Mark III was intended to compete against the Famicom juggernaut, but came up very short in that regard, despite the Mark III's superior graphics and CPU.
The Mark III was redesigned and sold outside Japan as the Sega Master System in 1986/1987, and was even rereleased in this form in the Japanese market in late 1987, albeit with slight differences. Sega discontinued the Mark III in 1989 along with the Japanese Master System, so that Sega could focus on the new 16-bit Mega Drive.