The 1991 United States Grand Prix was a Formula One motor race held on March 10, 1991 in Phoenix, Arizona. It was the first race of the 1991 Formula One World Championship. The 81-lap race was won from pole position by Ayrton Senna, driving a McLaren-Honda, with Alain Prost second in a Ferrari and Nelson Piquet third in a Benetton-Ford.

The McLaren MP4/6 is a Formula One racing car designed by McLaren's Neil Oatley, Matthew Jeffreys, David North, David Neilson, Bob Bell and Mike Gascoyne; powered by the Honda RA121E V12 engine for use in the 1991 Formula One season, with the engine's design and development led by Osamu Goto. It was driven by reigning World Champion, Brazilian Ayrton Senna, and Austria's Gerhard Berger. Ayrton Senna would win his third World Championship in the MP4/6. The MP4/6 was notable for being the last F1 car to win the championship with a manual gearbox and the only F1 car powered by a V12 engine to do so.