NFL Network's Ian Rapoport confirmed the deal prior to the official announcement, making it the third major move ahead of Tuesday's trade deadline, with Jimmy Garoppolo being traded from New England to San Francisco and offensive lineman Duane Brown being moved to Seattle from Houston.
The 24-year-old Ajayi is in the midst of his third NFL season after getting selected in the fifth round of the 2015 NFL draft out of Boise State.
He has rushed for 465 yards on 138 carries for a 3.4 yards-per-carry average this season. He has also made 14 catches for 67 yards but has yet to find the end zone.
Last season, Ajayi rushed for 1,272 yards and eight touchdowns, and he was named to the Pro Bowl.
With the acquisition of Ajayi, the Eagles will be able to utilize former starter LeGarrette Blount in a short-yardage role with Wendell Smallwood in a pass-catching capacity.
Jason La Canfora of CBS Sports reported the Eagles have no plans to move on from Blount after the deal and that he will "have a role."
The Eagles lead the NFC with a 7-1 record, and they are fifth in rushing yards per game with 129.2, but the addition of Ajayi gives them an even more dangerous rushing attack.
Miami is in the AFC playoff hunt at 4-3, and it will be forced to rely on Damien Williams and Kenyan Drake as its leading rushers, although they have just 22 combined carries this season.