Jo Siffert, born July 7, 1936 – died October 24, 1971, was a Swiss race car driver.

Born Joseph Siffert in Fribourg, Switzerland, he was the son of an automobile dealer. He initially made his name in racing on two wheels, powering his way to the Swiss 350cc motorcycle championship in 1959 before switching to four wheels with a Formula Junior Stanguellini. Affectionately known as "Seppi" to his family and close friends, Siffert graduated to Formula 1 as a privateer in 1962 with a four-cylinder Lotus-Climax , later driving under the Swiss Scuderia Filipinetti banner and in 1964 joined British private owner Rob Walker 's team. In 1968, Siffert drove into the F1 history books by winning the British Grand Prix at Brands Hatch in Walker's Lotus 49B, beating Chris Amon's Ferrari into second place after a race-long battle.

While Siffert's status in F1 grew slowly, his fame came as a leading driver for the factory Porsche effort in its quest for the World Sports Car Championship . Siffert's driving displays in the 917 prototype were legendary, earning him several major wins in Europe. In addition, Siffert was chosen by Porsche to help launch its Can-Am development program.

In 1970 he teamed up with Brian Redman to drive a Porsche 908/3 to victory at the Targa Florio. That same year, Porsche bankrolled Siffert's seat in a works F1 March since the German company did not wish to lose one of their prize drivers to rival Ferrari. His association with March was disastrous, so he was delighted to join Pedro Rodriguez at BRM the following season.

Jo Siffert was killed in an end-of-season non-championship F1 race at Brands Hatch, the scene of his greatest victory.