"It is some time since I said that the only Premier League club I would play for other than Manchester United was Everton, so I am delighted that the move has happened," Rooney said in a statement released to the UK's Press Association.
"Thirteen years ago I went to United with the intention of winning trophies and I have been fortunate to be a part of one of the most successful periods in the club's history.
Rooney won five English Premier League (EPL) titles plus the Champions League, Europa League and FA Cup while at United, making 559 appearances for the Red Devils.
He scored 253 goals, beating the previous best of the legendary Bobby Charlton last season.
Rooney broke Charlton's England scoring record in September 2015, taking his tally to 53 last summer, but he has dropped out of the national squad due to limited appearances at Manchester United.
Lukaku headed other way
Rooney's move comes a day after United announced it had agreed terms with Everton for the signing of their star striker Romelu Lukaku for a reported fee of $95 million.
Lukaku described the transfer as the "perfect opportunity" after scoring 25 EPL goals last season for the Toffees. The arrival of the Belgian international would have further limited Rooney's options, but he feels Everton has the potential to challenge for honors.
"I have come back to Everton because I believe (Everton manager) Ronald Koeman is building a team that can win something and I look forward to playing my part in making that a reality for the club I have supported since a boy," Rooney said.
Former Dutch international star Koeman is delighted to have secured the services of a player of Rooney's stature.
"Wayne has shown me that ambition that we need and that winning mentality -- he knows how to win titles and I'm really happy he's decided to come home," said Koeman
"He loves Everton and he was desperate to come back. He is still only 31 and I don't have any doubts about his qualities. It's fantastic he's here."
Rooney scored 17 goals in 77 appearances for Everton as a teenage prodigy, attracting the attention of Manchester United and its legendary manager Alex Ferguson.
He proved one of Ferguson's most successful signings, being the club's top scorer for five seasons, sharing the honor with Cristiano Ronaldo on another occasion.
But he made just 15 EPL starts last season although Mourinho continued to lavish praise on the player.
'It's no secret I've long been an admirer of Wayne," he told the club's official website.
"He's been a model professional and will remain in the history books for years to come.
"His experience, focus and determination will be missed and I wish him well for the future."
The EPL season starts on August 12, with Chelsea, also reported to have been interested in re-signing its former striker Lukaku, defending champion.