The 33-year-old forward, who is Manchester United and England's all-time record goalscorer, will join the Rams on an initial 18-month contract.
He signed for Washington-based DC from Everton in July 2018 and has scored 23 goals in 41 league games.
"I'm sure I can make a big contribution," said Rooney.
Rooney has agreed to join Derby despite having more than two seasons left on his deal with DC United.
"While the decision to move home was a tough one, family is everything to us and we make this change to be closer to the ones we love back in England," he added.
"I am very excited about the opportunity Derby County have provided me with. I am looking forward to joining [Derby manager] Phillip Cocu, his coaching staff and the first-team squad from the start of January.
"I am equally excited to begin my coaching career at Derby County, working with both the first team and academy."
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