The Mercedes-Benz M121 engine was a 1.9 liter single overhead camshaft inline four-cylinder engine introduced by Mercedes in 1955 and used in various model lines during the 1950s and 1960s. Originally rated at 56 kW at 5500 rpm, it replaced the 1.8 liter M136 introduced in 1935, offering improved performance over the M136's side camshaft design.

The Mercedes-Benz W120 and Mercedes-Benz W121 are technically similar inline-four cylinder sedans made by Daimler-Benz. The W120 was first introduced by Mercedes-Benz in 1953. Powered initially by the company's existing 1.8 liter M136 engine, it was sold as the Mercedes-Benz 180 through 1962. From 1954, Mercedes-Benz also offered the W120 with a diesel engine as the Mercedes-Benz 180 D. The Mercedes-Benz W121 was introduced as the Mercedes-Benz 190 in 1956, powered by a 1.9 liter M121 engine. From 1958, the W121 was also offered with an OM621 engine, sold as the Mercedes-Benz 190 D through 1961.