The Mercedes-Benz W123 is a range of executive cars produced by German manufacturer Mercedes-Benz from November 1975 to January 1986. The W123 models surpassed their predecessor, the Mercedes-Benz W114, as the most successful Mercedes, selling 2.7 million units before production ended in the autumn of 1985 for the saloon/sedan versions and January 1986 for coupés and estates/station wagons.

The M123 engine family was a straight-6 automobile engine from Mercedes-Benz, used from 1976 to 1984 in the W123 series models 250 and 250 T. It replaced the 6-cylinder engines M180 in the 230 and M130 in the 250 versions from the W114 series.