Carr, the No. 1 overall pick in the 2002 NFL draft, tested the free-agent market before returning to back up Eli Manning for the third straight year. This will be the fifth season in the last six years that he has been the Giants' backup.
The Giants and Manning are comfortable with Carr, who provides starting experience and has played a key role as the scout team quarterback in practices.
"I like the room with he and Eli and whoever else is in there," coach Tom Coughlin said in a team release. "Don't forget, there's another side to David Carr. David Carr can run, he's fast, he's athletic. No matter who we're going to play, he can be the opposition's quarterback and do it very well. He does a good job of it. He runs the option."
That's important for the Giants considering that the Redskins and Eagles both feature option schemes in the NFC East.
Carr, who played in San Francisco in 2010, doesn't see much action behind Manning. In his four seasons with the Giants, Carr has played in 11 games and completed 32 of 48 passes for 359 yards and three touchdowns.
Manning has started 135 consecutive regular-season games, the third-longest quarterback streak in NFL history.
The 6-foot-3 Carr, who turns 34 in July, joins Manning and Curtis Painter as the quarterbacks on the Giants' roster.