The Atari Microsoft BASIC and Atari Microsoft BASIC II variants of the BASIC programming language were cartridge or floppy disk packaged versions of the industry standard Microsoft BASIC dialect adapted to the Atari 800 and later Atari computers of that architecture.

Atari Microsoft BASIC differed from standard Microsoft BASIC of the day in that the graphic coordinates had the Y-axis going up the screen, rather than the more popular down. This complicated users copying programs from common sources such as Creative Computing magazine.

The cartridge model number for Atari MS BASIC was CX8126, and version II was model RX8035. Although in many regards more capable than Atari's in-house¹ ATARI BASIC aka Sheperdson BASIC, Microsoft BASIC was never a definite success on the Atari 8-bit platform.

( ¹ ATARI BASIC was in fact developed by an external contractor team but branded as and sold by Atari themselves )