The transfer was made official via United's Twitter account:
Per football writer Kristan Heneage, United confirmed the deal was for three-and-a-half years. Rooney is scheduled to make his debut on July 14 against Vancouver Whitecaps, the first at the brand new Audi Field venue.
According to Rick Maese and Steven Goff of the Washington Post, Rooney will earn a salary of $13 million (£10 million) with his new club.
The transfer brings an end to Rooney's second spell with his boyhood club. The 32-year-old joined Everton in the summer of 2017 from Manchester United.