W123 is the internal chassis-designation Mercedes-Benz used for their executive line of cars, manufactured between 1976 and 1985. The W123 models surpassed their predecessor, the W114 and W115 models, as the most successful Mercedes, selling 2.7 million cars before replacement by the W124 after 1985. The additional range of smaller Mercedes-Benz W201 models were introduced in 1982.

Mercedes-Benz introduced the W123 four-door versions in January, 1976. While there were some technical similarities to their predecessors, the new models were larger in wheelbase and exterior dimensions. The styling was also updated, although stylistic links with the W114 / W115 were maintained. All W115 engines were carried over, with the 3 litre 5-cylinder diesel model being renamed from "240D 3.0" to "300D" (as it had already been called before in North American markets). The only new engine was the 250's 2,525 cc inline-six (Type M123, a short-stroke version of the 2.8 litre six Type M110) that replaced the old 2,496 cc Type M114 "six".

In the spring of 1976, a coupé version was introduced on a shorter wheelbase than the saloon (2,710 mm (106.7 in) versus 2,795 mm (110.0 in)) for the saloon). This W123C/CE was available as a 230C (later 230CE) and as a 280C/CE in most markets; in North America there were additional 300CD versions with naturally aspirated, later turbocharged 3 litre diesel engines.