Rivers, the No. 9 overall pick out of USC in 2008, missed all of last season with a wrist injury and has yet to play a full season because of injuries.
It's a low-risk deal for the Giants, who hope to improve at linebacker by dealing the No. 32 pick in the fifth round for a former top-10 pick who hasn't reached his potential. The Giants also have two fourth-round choices this year due to a league-awarded compensatory pick.
"I'm excited to get a fresh start and brand new beginning," Rivers said in a statement released through the Giants. "You can't ask for more than to be playing for the defending world champions and be in New York.
"I just haven't been able to stay on the field as much as I would like," he added. "Hopefully, it's a new beginning and everything is behind me and I'm ready to do it on the big stage here in New York."
The Rivers trade was the headliner on a busy two-day stretch for the defending Super Bowl champions.
New York also added depth to its offensive line and secondary by signing former Washington Redskins and Seattle Seahawks offensive lineman Sean Locklear and former New England Patriots cornerback Antwaun Molden on Wednesday.
Locklear has played in 105 regular-season games with 82 starts in eight NFL seasons, including starting all 19 games at right tackle during the Seahawks' run to the Super Bowl in 2005.
"I prefer right (tackle)," Locklear said in statement released by the team. "That's the position I played the most."
"I talked to all of the coaches -- the offensive line coach (Pat Flaherty), coach (Tom) Coughlin. They said, 'Come and compete, just like everybody else. Nothing is handed to you.' "
The Giants have tried to fortify their secondary by re-signing Terrell Thomas, Bruce Johnson, Michael Coe and Justin Tryon while adding Molden and safeties Stevie Brown and Chris Horton in free agency.
The Giants want to create competition at as many positions as possible and Rivers, who turns 26 in May, will have to come in and prove himself. He played just seven games his rookie season due to a broken jaw he suffered on a blindside block by former Steelers receiver Hines Ward. Rivers also missed three games in 2009 due to a calf injury but finished with 72 tackles and a sack. He played through a plantar fasciitis foot injury in 2010, appearing in 15 games and making a career-high 77 tackles and registering a sack.
"This is an excellent opportunity for him to have a fresh start," Coughlin said. "Keith is versatile enough to play the outside on first and second down and he can play on third down because he runs well. And he's fast enough to rush the passer."
General manager Jerry Reese has mentioned outside linebacker Michael Boley as a possibility at middle linebacker for the 2012 season. Boley was the team's best every-down linebacker and spent much of last season making calls in the huddle and emerging as a leader on the defense after Jonathan Goff was lost for the year with an ACL injury prior to the season opener.
Goff remains an unrestricted free agent.
Defensive coordinator Perry Fewell often used three safeties over the past two seasons but Rivers gives Fewell options with a linebacking corps that includes Boley, Mathias Kiwanuka, Jacquian Williams, Greg Jones, Mark Herzlich, Clint Sintim and Spencer Paysinger. Kiwanuka could potentially play more defensive end going forward after playing in a hybrid role last season.
"We're excited to have another talented player in the mix for our defensive staff to work with," Reese said. "We try to create competition at every position, and Keith will definitely bring some competition to our linebacker group."