The Giants and Manningham agreed Tuesday to a one-year contract, a source told ESPN NFL Insider Adam Schefter. The team announced the move in a press release.
"I'm happy to be back," Manningham said in the release. "I'm real excited. It's just great to see everybody, be around familiar faces, knowing everybody in the past and just being comfortable around everybody. Once a Giant, always a Giant. That's how I feel. This is a very special place. I wanted to come back."
Manningham, 27, struggled through an injury-plagued 2013 season with the San Francisco 49ers, making just nine receptions for 85 yards in six games.
"It was frustrating, but it was a learning experience also," Manningham said. "When I got back I couldn't do what I wanted to, I knew I needed a little bit more time. It's coming along and I just have to work on getting stronger and getting my strength back and trusting it more. I'm doing good; it's getting stronger."
Prior to his two-year stint in San Francisco, Manningham played his first four NFL seasons with the Giants.
Manningham was an integral part of New York's improbable championship run following the 2011 season, catching touchdown passes in each of the Giants' three NFC playoff victories and making a key fourth-quarter reception along the sideline in the Super Bowl against the New England Patriots.
The 6-foot Manningham has 211 catches for 2,849 yards and 19 touchdowns in six seasons with the Niners and Giants.
The acquisition of Manningham marks the latest in what has been an extremely busy offseason for the Giants. The team already had added defensive backs Dominique Rodgers-Cromartie, Walter Thurmond and Quintin Demps, running back Rashad Jennings, offensive lineman Geoff Schwartz and kick returner Trindon Holliday in free agency.