Rooney left Derby County earlier this month and then went back to the MLS team.
After bringing in Miguel Berry earlier, he has now added Morrison via a free transfer.
Morrison’s contract has a termination date of 2023, although the club has the option to extend it by another year.
“Ravel is a dynamic and exciting player with experience performing at the best levels in Europe and Mexico,” said DC United General Manager Lucy Rushton.
He is an elite midfielder with great technical skill and positional diversity. He can seamlessly fit into the system and playing style that Wayne wants to employ thanks to his game intelligence and all-around attacking abilities. We are eager for him to join the roster and start making an impact on the field as soon as feasible.
At Pride Park last season, the 29-year-old played with Rooney and registered five goals across all competitions.