Van der Sar, who played in the Netherlands' 2-0 World Cup qualifying win over Romania on Saturday, has signed a two-year contract beginning on July 1. The transfer fee was not disclosed.
The 34-year-old, who is five caps short of a century of international appearances, joined Fulham from Juventus for 5 million pounds in 2001 and has been a consistent performer for club and country.
United have struggled to find a long-term replacement for Danish goalkeeper Peter Schmeichel who left Old Trafford in 1999.
Australian Mark Bosnich and Frenchman Fabien Barthez left the club after failing to impress manager Alex Ferguson, and Roy Carroll and American Tim Howard shared the goalkeeping duties last season.
United released Northern Ireland's Carroll and Spaniard Ricardo, their third-choice keeper, last month.