The Motorola 6809 ("sixty-eight-oh-nine") is an 8-bit microprocessor with some 16-bit features. It was designed by Motorola's Terry Ritter and Joel Boney and introduced in 1978. Although source compatible with the earlier Motorola 6800, the 6809 offered significant improvements over it and 8-bit contemporaries like the MOS Technology 6502, including a hardware multiplication instruction, 16-bit arithmetic, system and user stack registers allowing re-entrant code, improved interrupts, position-independent code and an orthogonal instruction set architecture with a comprehensive set of addressing modes.

Among the most powerful 8-bit processors of its era, it was also much more expensive. In 1980 a 6809 in single-unit quantities was $37 compared to $9 for a Zilog Z80 and $6 for a 6502. It was launched when a new generation of 16-bit processors were coming to market, like the Intel 8086, and 32-bit designs were on the horizon, including Motorola's own 68000. It was not feature competitive with newer designs and not price competitive with older ones.

The 6809 was used in the TRS-80 Color Computer, Dragon 32/64, SuperPET, ENER 1000, and Thomson MO/TO home computers, the Vectrex game console, and early 1980s arcade machines including Star Wars, Defender, Robotron: 2084, Joust, and Gyruss. 1990s Williams pinball machines are equipped with WPC-series controller boards based on 68B09. Series II of the Fairlight CMI digital audio workstation and Konami's Time Pilot '84 arcade game each use dual 6809 processors. Hitachi was a major user of the 6809 and later produced an updated version as the Hitachi 6309.