The Williams FW20 was the car with which the Williams Formula One team competed in the 1998 Formula One World Championship. It was driven by Jacques Villeneuve, the reigning champion, and Heinz-Harald Frentzen, who was in his second year with the team.

1998 was very disappointing compared to the previous championship-winning campaign. The team endured their first winless season since 1988 and finished a distant third in the Constructors' Championship, with three podiums and no wins, a long way behind McLaren and Ferrari. Villeneuve was unhappy with the season and had no chance to defend his title. He signed with the new British American Racing team for 1999, and Frentzen departed to Jordan after two seasons spent largely in the shadow of his teammate.