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.
Per MLS' official website, Rooney expressed his delight at joining his new club:
"It is fantastic to be joining D.C. United at such an exciting time in the club's history with the new stadium opening in just a few weeks. Moving to America and MLS fulfills another career ambition for me. I have the hunger to be a success here and will give D.C. 100 percent—as I have always done for every team I have ever played for.
"When I visited earlier this summer, I was really impressed with everyone I met connected with the club, and of course the new Audi Field. Now I can't wait to get on the pitch in a United shirt and join my new team-mates to bring success to this club."
The club's managing general partner and CEO Jason Levien also weighed in on the move:
"This is a seminal moment for our fans and organization. Wayne is a global soccer icon and his presence at D.C. United will elevate our product on the pitch and soccer as a whole in our city and in this country. Wayne has thrived when competing at the most elite levels of soccer and we're thrilled to have his leadership as we enter this new era at Audi Field."
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.
In the 2017-18 campaign, Rooney, who is the Red Devils' and England's all-time record goalscorer, finished as the Toffees' top scorer in the Premier League with 10 goals and had some moments to remember. He scored on his second debut against Stoke City, while also netting against Manchester City and Liverpool.
Tony Scott of the Liverpool Echo summed up the impact the former Manchester United man had on the team last term:
Tony Scott @Tony_Scott11
EFC stats 17/18 Wayne Rooney EFC Team ranking- 1st - Goals scored (10 goals from 27 starts) 2nd - Chances created (Sigurdsson 1st) 1st - Shots 1st - Shots on target 2nd - Key passes per game (Sigurdsson 1st) 1st - Goals involved in. EFC failed to win a PL match without Rooney
However, most of the good work done by Rooney came in the first half of the campaign, and following the midseason appointment of Sam Allardyce, he was dropped into a deeper role.
While there were times when Rooney performed well in a midfield position, in the main he appeared laboured and was wasteful in possession. He didn't create or score another goal for Everton from December and was given the runaround by energetic opposition at times.
Statistician Paul Carr broke down the good and bad aspects of Rooney's game during his one year back at Goodison Park:
With Everton seeking to go in a different direction under new manager Marco Silva, an exit appears to suit both parties. The high-intensity, expansive style of football the Portuguese likes his teams to play isn't tailored to the strengths of Rooney.
The United and England legend will no doubt be looking forward to a fresh challenge in Washington, though as Paul Hirst of The Times noted, it's unlikely to be a glamorous venture to the United States to begin with:
DC United have only won two of their 12 games in the 2018 Major League Soccer campaign. They've also played at least three fewer matches than the rest of the sides in the Eastern Conference, as work continues on their new stadium, Audi Field.