The signing gives the Giants a talented outside receiver to play opposite Odell Beckham Jr. New York used
The Jets released Marshall less than a week ago, making him eligible to sign with any team before the start of free agency on Thursday.
Marshall, who will turn 33 this month, had 59 catches for 788 yards and three touchdowns last season.
Marshall said the Giants' offer might have been the lowest that he got after being released by the Jets last week.
However, Coach Ben McAdoo's team, which made the playoffs with an 11-5 record last season, offered him something else.
"It was all about championships," said Marshall, who has never made the playoffs since being drafted in the first round by Denver in 2006. "To get straight to the point — it was all about a championship, they presented the best opportunity to be in a championship organization."