Financial terms were not disclosed, but agent Drew Rosenhaus told ESPN's Adam Schefter that the deal is worth up to $3 million.
The Giants placed wide receiver Sterling Shepard on injured reserve in a corresponding move. Shepard, who has turf toe, was injured during Sunday's 17-13 loss to the Chicago Bears. He will have to miss at least three weeks, per IR rules for this season. He has eight receptions for 76 yards.
Barkley tore the ACL in his right knee during Sunday's loss. Dion Lewis played every snap in his absence.
"Devonta turned down more money from other teams because he likes the opportunity with the Giants best," Rosenhaus told Schefter.
Freeman, 28, spent the first six years of his career with the Atlanta Falcons. He has two 1,000-yard rushing seasons (2015 and '16) and has averaged 4.2 yards per carry in his career.
Freeman gained 656 yards rushing with two touchdowns on 184 carries (3.6 average) last season. The Falcons let him walk as a free agent this offseason and instead signed Todd Gurley.
"Wayne and Dion are definitely guys that will factor in," coach Joe Judge said Monday. "We'll work to their strengths -- they are two different types of backs. Both of them have a degree of toughness. Wayne's got that long speed, get him ranging out and get him really moving. Dion's got that real good short-area quickness and burst to get going. He does a great job of finding small seams and getting through the hole. They are both different guys; we're going to have to work with what they do well to give them an opportunity to be successful."
Now the Giants add Freeman, who visited the team on Tuesday, to the mix.
Barkley was placed on injured reserve earlier Tuesday. Cornerback Ryan Lewis was added to the active roster.
Meanwhile, the loss of Shepard puts more on the plate of wide receivers Golden Tate and Darius Slayton. Tate returned last week from a hamstring injury and took Shepard's place in the second half against the Bears.
But with the loss of Barkley for the season and Shepard for at least three games, the Giants will be searching for explosive plays. Some of the onus will fall on tight end Evan Engram, who has run 44 of his 73 routes this season out of the slot. Engram's longest catch this season is for 22 yards.