The Williams FW31 is a Formula 1 car used by Williams in the 2009 season. The car was presented on 19 January 2009 at the Autódromo Internacional do Algarve. Nico Rosberg battled for the points all season in this car, while Kazuki Nakajima failed to score any points. As a result, the team only finished seventh in the Constructors' Championship.
At the Australian Grand Prix there was a fuss about the double-deck diffuser with which the car was equipped, just like the Brawn BGP 001 and Toyota TF109. An official complaint was made, but the cars were declared legal by the FIA.