The Mercedes-Benz W126 is a series of passenger cars made by Daimler-Benz AG. It was marketed as the second generation of the Mercedes-Benz S-Class, and manufactured in sedan/saloon (1979–1991) as well as coupé (1981–1990) models, succeeding the company's W116 range. Mercedes-Benz introduced the 2-door C126 coupé model, marketed as the SEC, in September 1981. This generation was the first S-Class to have separate chassis codes for standard and long wheelbases (W126 and V126) and for coupé (C126).

Over its 12-year production (1979–1991), 818,063 sedans/saloons and 74,060 coupés were manufactured, totaling 892,123 and making the W126 by far the most successful generation of S-Class to date, and the longest in production.

After the debut of the W116 S-Class in 1972, Mercedes-Benz began preparing for the next generation S-Class, in October 1973. The project, code-named "Project W126", aimed to provide an improved ride, better handling, and improved fuel efficiency. The oil crisis of 1973 and increasingly stringent emission and safety regulations in the United States had an important influence in developing the W126 for reduced emissions and increased fuel efficiency.

The W126 design team, led by Mercedes-Benz's chief designer Bruno Sacco, aimed to design a more aerodynamic shape and retain the unmistakable S-Class design elements. The aerodynamic drag was reduced through lengthy wind tunnel testing and reshaping of the front end and bumper along with hiding the wipers underneath the hood/bonnet for smoother flow. Reducing the weight was accomplished by extensive use of high-strength low-alloy (HSLA) sheet and polyurethane deformable material for bumpers and side claddings. The lighter alloy material was used for the heavily revised M116/M117 V8 engines for reduced weight. Both contributed to the reduction of fuel consumption by 10% as compared to its predecessor.

After six years of development, the W126 was introduced at the IAA Frankfurt in September 1979. At the introduction, S-Class was available in two wheelbase lengths (standard and long) and three petrol engine options with one six-cylinder inline engine and two V8 engines. The diesel engine option was introduced in September 1981 exclusively for the North American market.

At the 1981 IAA Frankfurt, the C126, a coupé version of the S-Class, was introduced with 380 SEC and 500 SEC trims initially offered. It was the first time that a coupé version was derived from an S-Class chassis. However, the styling was different between the two, making it the first time that the coupé had fully distinct styling, later evolving into CL-Class. Carrying on the tradition of past Mercedes coupé models, the C126 featured a pillarless design with no B pillar between the front and rear side windows. At the same time, the "Energiekonzept" (Energy Concept) was introduced to improve the fuel efficiency of S-Class through engine revisions.

The W126 was revised in 1985 for the 1986 model year. The revised "Second Series" model range was introduced at the 1985 IAA Frankfurt with new six-cylinder petrol and diesel engines and the V8 petrol engines enlarged to 4.2 and 5.5 litres. The 5-litre V8 was carried over. Visual changes included smoother bumpers and side claddings, revised "Gullydeckelfelge" (German for manhole-cover wheel rim) alloy wheels, and a deeper front bumper with integrated air dam.

The W126 generation was replaced by the W140 in 1991.