The BMW M21 is a straight-six diesel engine developed by the Bavarian engine manufacturer BMW. It has swirl chamber injection and is based on the M20 petrol engine and was produced for BMW by the Upper Austrian Steyr engine plant from 1983 to 1991. It was succeeded by the M51.

The BMW M20 is a SOHC straight-six petrol engine which was produced from 1977 to 1993. It was introduced eight years after the larger BMW M30 straight-six engine, which remained in production alongside the M20.