The Giants signed Koo to the practice squad Tuesday, Ian Rapoport of NFL Network reports.
Koo was released by the Falcons on Friday after going 2-for-3 in Week 1 against the Buccaneers before not playing Week 2 against the Vikings. He'll get another opportunity with the Giants and could be elevated to the active roster for Sunday's game against the Chargers if Graham Gano (groin) is unable to play.