Rooney, 33, will join the Rams on an 18-month contract with the option of a further year after spending 13 months in the United States.
The former Manchester United and England captain made a strong impact in MLS, scoring 23 goals in 44 matches.
However, his desire to get into management has brought about a return to England, linking up with Cocu in a position that will see him develop as a coach and continue playing.
"I am very excited about the opportunity that Derby County have provided me with," Rooney told the club's website.
"I am looking forward to joining Phillip Cocu, his coaching staff and the first-team squad from the start of January.
"I am sure I can make a big contribution and can't wait to meet everyone, especially the supporters.
"I must also take this opportunity to thank DC United, their owners, executives and supporters who have been superb to work with. I want them to know that I will give my all in support of our push for the play-offs."
Cocu said: "Wayne Rooney is an extremely talented and top-class footballer.
"It is an exciting prospect for him to be joining Derby County and he can bring so much to this squad, both on and off the pitch. He has enjoyed a wonderful career, both in club football and for England, and he still has so much to give as well.
"His credentials speak for themselves. But to have a player with his leadership, skill, experience, character and work ethic will be huge for us. He knows what it takes to succeed as a player and I have no doubt he will have a positive impact right across the club."
Rooney's last season in England did not go entirely to plan, as he often appeared off the pace in the Premier League with Everton, despite scoring 10 goals in 31 games.
Previously he became United's all-time leading scorer in 2017, surpassing Bobby Charlton's record of 249 to cement his name in the club's history.
He also won a host of trophies in his time at Old Trafford, including the 2007-08 Champions League and five Premier League titles, and became club captain.
Rooney is also the most-capped outfield player for England, having played 120 times for his country and scored a record 53 goals.
Derby's primary goal this season will be to return to the Premier League from the Championship, having lost in the play-off final to Aston Villa last term.