The Williams FW31 is a Formula One motor racing car, designed and built by WilliamsF1. The AT&T Williams team used the FW31 to compete in the 2009 Formula One season. The car was unveiled on 19 January 2009 at the Autódromo Internacional do Algarve circuit in southern Portugal, and was first driven by the team's test driver Nico Hülkenberg. It was a mid-field runner, in contention for points on many occasions when driven by Rosberg, but rarely contending for podiums. Rosberg could have finished in third position at Marina Bay during the 2009 Singapore Grand Prix if he had not run wide on the exit of the pit lane and received a penalty.

Rosberg was criticised for failing to achieve a pole position while topping the times in practices on numerous occasions, notably at the 2009 Brazilian Grand Prix - the Williams cars had excelled during practice sessions in wet conditions, but did not produce the same level of performance in qualifying. While Rosberg often scored points, and elevated the car to a finishing position that it was considered incapable of, Nakajima failed to score a point all season and possibly cost the team a higher position in the championship standings than the seventh place they eventually achieved.

At the first race of the season, an official complaint was launched by other teams against the rear diffusers of the Williams FW31, Toyota TF109 and the Brawn BGP 001 saying that they were illegal, but after analysing the cars the FIA reported that the cars were not illegal. The teams appealed and after much deliberating the car were deemed legal by the FIA.

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