The Lotus 43 was a Formula One racing car designed by Colin Chapman for the 1966 season. Hampered by its heavy and unreliable BRM engine, it won only one race, the 1966 United States Grand Prix.

The Lotus 43 was partially based on the Lotus 38 Indycar, due to Chapman's experience at Indy with larger engine capacity and tyre/suspension setup. The car was designed in this way in response to new regulations which came into force in 1966, which increased the engine capacity to 3 litres. Along with newer, wider tyres better able to handle the power of the larger engines, the need for a more robust design was obvious.

Cosworth were developing a new engine for Lotus, the DFV, to be introduced for the 1967 Formula One season, and in the meantime Chapman made a deal for use of BRM's new P75 H16 engine. The P75 on paper was technically advanced and powerful, and Chapman had hopes that it would power his cars to another successful season.

The first sign of trouble was when the new engine arrived and it required four men to lift it from the truck. The engine proved to be overweight, unreliable and was unable to produce the promised power. The car was supposed to debut at the 1966 Monaco Grand Prix, driven by Peter Arundell, but it was unavailable. Its first outing at the following Belgian Grand Prix ended during practice, when the engine gave out. Both Clark and Arundell then reverted to using the Lotus 33 while the new car's problems were ironed out.

The 43 reappeared at the Italian Grand Prix but retired with gearbox failure. Clark then won the United States Grand Prix at Watkins Glen, the only race win for the P75 engine and borrowed the P75 engine before the race. However, gearbox failure again led to the 43's retirement from the final race of the season, the Mexican Grand Prix.

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