The 33-year-old is on holiday in Barbados following the end of the MLS season, having spent the past year-and-a-half with DC United.
DC were thrashed 5-1 by Toronto in the MLS play-offs in Rooney’s final game for the club.
The former Manchester United striker will join Derby as a player-coach on an 18-month deal from January.
And The Mirror claim the Championship club will order Rooney to complete a full pre-season programme before making his debut.
Rooney, who scored 25 goals in 52 appearances in the United States, will become the highest paid player in the Championship on a reported £100,000-a-week deal.
He said upon signing: “I am very excited about the opportunity that Derby County have provided me with.
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“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 am equally excited to begin my coaching career at Derby County working with both the first-team and academy.”
Derby are 15th in the Championship after winning just three of their 12 league games under new manager Cocu.