The Vikings announced the deal on their website. They will receive a pick in next year's draft and a conditional selection in the 2012 draft.
The deal is contingent on both players passing physicals on Saturday.
Rosenfels will back up Eli Manning. The Giants lost backup Jim Sorgi with a shoulder injury and he was placed on injured reserve, leaving untested Rhett Bomar as the only other quarterback.
The Vikings acquired Rosenfels from Houston in a trade before last season and signed him to a two-year deal worth $9 million to compete with Tarvaris Jackson for the starting job.
At the time, Rosenfels was thrilled to be coming to a team giving him a legitimate shot at winning the job for the first time. After an offseason of working out and learning the system, however, Rosenfels' big shot was dashed when Favre waltzed in to take over.
Rosenfels spent all of last season as the No. 3 quarterback behind Favre and Jackson.
"If it's true sad to c you go sage good qb great teammate," Vikings receiver Greg Lewis tweeted on Friday night. "Good luck in new York."
There was a brief glimmer of hope for Rosenfels this summer when Favre was considering retirement. Once again, that ended after the first preseason game, when the Vikings flew three high-profile teammates to Mississippi to persuade Favre to return for one more year.
Time was running out for the 32-year-old Rosenfels, who clearly grew more frustrated with his position on the team.
He had an impressive preseason opener at St. Louis, completing 23 of 34 passes for 310 yards and three touchdowns. But he was just 8 for 17 for 92 yards with one TD in the next three games combined, and coach Brad Childress said after the preseason finale that Jackson would be Favre's backup again this season.
The move means that the Vikings likely will keep rookie Joe Webb as the third quarterback.
Sorgi signed with New York as a free agent after six seasons as Peyton Manning's backup in Indianapolis.
Reynaud is a converted receiver who was trying to make the Vikings as a running back and return man. He was the team's primary punt returner during the preseason.