The M20 is an inline-6 piston engine by BMW.

Initially designated M60, the 12-valve, belt driven SOHC design was introduced in the 1977 BMW 520/6 and 320/6 as an entirely new design. With displacements ranging from 2.0 to 2.7 liters, it was the "little brother" to the larger BMW M30 engine. It had 91 mm (3.6 in) bore-spacing instead of 100 mm (3.9 in) of the M30. It was intended to replace the larger displacement four-cylinder motors and was born out of BMW's conviction that a small six had more development potential than a large four (i.e. 2 liters+)

Powering the E21 and E30 3-Series, as well as E12, E28 and E34 5 Series cars, it was produced for nearly two decades, with the last examples powering the E30 325i touring built until April 1993. By that time, the newer twin-cam M50 engines (which shared the dimensions of the M20B20 and -B25) with four valves per cylinder had already been used in the E36 and E34 for a couple of years.

Three different head castings were used over the engine's production run. The earliest was #1264200 aka the "200". These were used in all e21 320/6 and 323i and e12 520/6 engines and later in the e28 and e30 eta engines (eta denoting the 'efficiency' version of the engine, with a lower engine redline amongst other focused differences aimed at increasing fuel economy). The next version was #1277731 aka the "731". This head was the same as the 200 but featured larger intake ports. The final version was #1705885 or "885" introduced in the 325i. Ports were further enlarged, valves were larger and the combustion chamber was redesigned to improve flow and thermodynamic efficiency.