Grant, 32, was a free agent and waiting at home for the Giants to ring him up. He reportedly turned down an offer from the rival Philadelphia Eagles.
The Giants released DB Cary Harris to clear room on the roster for Grant.
Grant told reporters that he wanted to come back to the Giants because he has 'unfinished business' with the Eagles.
He also reclaimed his #34 jersey. The Giants needed some depth at safety. All they really have back there are Phillips, Antrel Rolle and rookie Tyler Sash...