The record goalscorer for both England and Manchester United signed an 18-month contract with Derby and will officially join the second tier club in January 2020 once he has finished the MLS season.
Derby's interest in the former United star emerged shortly before the Rams, under new boss Phillip Cocu, opened their Championship campaign with a 2-1 victory at Huddersfield on Monday.
Reports suggested Rooney's wife Coleen was homesick and had returned to Britain with their children.
Rooney conceded he was keen to come back to England but he is working to secure his coaching qualifications and insisted the chance to learn from former PSV Eindhoven boss Cocu was a major motivation for his unexpected move.
"The opportunity to come back to England and play, but also to take up a coaching role with the club and work under Phillip was too great for me to turn down," Rooney said at a press conference on Tuesday.
"First and foremost I'm coming to play and to help the team, and second I want to learn off Phillip and his staff, to gain experience for when I do stop playing and go on to that next step.
"There were other clubs making offers and enquiring about me but I made the decision to come here and that was it, I wasn't going to go back on my decision."
The 33-year-old hopes his time at Pride Park is another step towards eventually becoming a manager, with a representative of the Football Association due to travel out to the United States next week to help his coaching progress.
Rooney is the eighth member of England's Euro 2004 squad to go into coaching or management after Frank Lampard, who is now in charge at Chelsea following a year at Derby, Rangers boss Steven Gerrard and Macclesfield manager Sol Campbell.
Gary Neville flopped in a short stint with Valencia, his brother Phil is manager of the England Women's team and John Terry is assistant to Dean Smith at Aston Villa, while Paul Scholes had a brief, ill-fated spell as Oldham boss last season.
"I think it's great to see them going into management but it's not just about what they're doing now," Rooney said.
"This is something I always had as an ambition to do. It's great these young English managers are getting opportunities.
"Over the last 15 or 20 years we've not seen so many great ex-England players coming through so it's great to see them doing that.
"It's something I always wanted to do when the time is right so hopefully it's something I can move into."
Cocu said Rooney's experience would be a huge boost to the club as they seek to return to the Premier League for the first time since 2008.
"The inspiration and motivation this can generate is massive for all of our players from the first team right through to the youngsters in the academy," he said of Rooney, who has scored 23 times in 43 DC United appearances since he joined from Everton on a three-year contract last year.
Derby have signed a lucrative new shirt sponsorship deal on the back of Rooney's signing, with the income helping to pay for his reported £100,000 ($122,000) per-week wages.
Rooney's return to England comes in the twilight of a glittering career that established him as an icon for club and country.
His meteoric rise started in August 2002 when he made his debut for Everton aged just 16, becoming the Premier League's youngest scorer with a superb long-range goal against Arsenal before his 17th birthday.
During 13 years with United, whom he joined in 2004, Rooney won the Premier League five times, the Champions League, the FA Cup and three League Cups.
Rooney is the second-highest scorer in Premier League history with 208 goals.
He also broke Bobby Charlton's record of 49 England goals, finishing with 53 from 120 appearances, but his time with the Three Lions is largely remembered for his disappointing performances in major tournaments.