The move was confirmed Monday by Trump lawyer Jay Sekulow, who said diGenova would begin work later this week.
"I have worked with Joe for many years and have full confidence that he will be a great asset in our representation of the president," Sekulow said.
The appointment of diGenova, who himself has offered strong rebukes of the Mueller investigation, likely signals a more aggressive push by Trump's legal team that had once pledged full cooperation with Mueller's investigators.