Redd is the most high profile Nittany Lions football player to leave the program after the school in the wake of the Jerry Sandusky child sex abuse scandal.
Redd is eligible to transfer immediately without sitting out a season after Penn State was hit with unprecedented sanctions two weeks ago, including a four-year bowl ban. Several Penn State players, including linebacker Michael Mauti and defensive lineman Jordan Hill, said last week they are staying. Redd was not among that group.
"We welcome Silas Redd to the Trojan Family," USC athletic director Pat Haden said in a statement. "He is an outstanding student and athlete. When the NCAA presented the option to transfer, Silas and his family put a lot of thought and research into making this decision.
"At USC, we've seen both sides of this issue, having lost a number of players to transfer due to our NCAA sanctions in 2010. But Lane Kiffin and his coaches would not be doing their job if they did not try to improve our team every single day. There is a specific need here for a player like Silas Redd, so Lane and our coaches recruited him within the guidelines set up in this instance by the NCAA."
Redd has two seasons of eligibility remaining. He ran for 1,241 yards and seven touchdowns last season, and ran for more than 100 yards in five consecutive games in mid-October. He was a second-team All-Big Ten pick.
Earlier this year, he was named to the Walter Camp and Doak Walker awards watch lists.
Sources told ESPN on Monday that Redd was leaning toward transferring to USC after his weekend visit to Los Angeles, where he met with USC coaches and players. Among his hosts were quarterback Matt Barkley and wide receiver Robert Woods.
Redd's only options were to stay at Penn State or go to USC, because of the unique nature of the opportunity at USC, a source close to Redd told ESPN's Joe Schad. Redd was intrigued by the chance to win big. He also was concerned that winning quickly might not be possible at Penn State because of players possibly leaving the program. Penn State was the only school Redd wanted to play for at one time.
USC is not allowed to have more than 75 scholarship players because of sanctions the school received in 2010 due to improper benefits. The school needs a spot for Redd, but USC believes at least one player on scholarship is academically ineligible. If not, a walk-on player likely would give up a scholarship.
Haplea, a 6-foot-4, 248-pound native of New Jersey, played in every game during the 2011 season for the Nittany Lions. He played a big role in helping Redd achieve a 1,000-yard season on the ground last year.
Haplea caught three passes for 21 yards and a touchdown in 2011.
Out of high school, Haplea was rated a three-star prospect by ESPN and the No. 18 tight end overall for the class of 2010. The incoming junior has not redshirted and will have three years to play two at Florida State.
Penn State coach Bill O'Brien and his staff have been working hard to keep the team together despite the sanctions, but Redd was his best offensive player to leave. Backup Bill Belton is a converted wideout.
Redd voiced support for former coach Joe Paterno as late as two weeks ago after former FBI director Louis Freeh released the results of his investigation in the Sandusky scandal for the university.
Freeh said Paterno, who died in January, and three other school officials concealed allegations against Sandusky -- conclusions vehemently denied by Paterno's family and the officials.
"It has nothing to do with us," Redd had said about the findings. Penn State later handed the results of Freeh's investigation to the NCAA.
Redd also said his opinion of Paterno, the coach that recruited him to Penn State, hadn't changed. He said Paterno, in his view, remained "the best college football coach of all time."
The only restriction on players leaving Penn State is they cannot practice or play with Penn State this year and still play for another school this season, meaning the Penn State roster should finally be set once training camp starts in a week. But the process has set up a college version of NFL free agency, in which other schools have been busy trying to cherry-pick Nittany Lions.
With an offense led by Barkley, the Trojans will be highly ranked and one of the favorites to win the Pac-12 Conference.
Most Penn State players interviewed after a rally and voluntary workout early Tuesday morning said they hoped Redd and others would stick around, but would honor their decisions regardless.
"Each player came here for different reasons and with different objectives," tight end Garry Gilliam said. "When it comes down to it, I'd like them to stay, but if they don't, I'll respect their decisions."
Penn State athletic director Dave Joyner wished Redd and the other transfers well. "I think that certainly we understand and it's within their purview," Joyner said in an interview Tuesday night with The Associated Press at an evening football function.
He added the low number of transfers was "a great testament to Bill O'Brien, and the kind of person he is, the kind of coach he is and the kinds of players these are overall.
"This team has a lot of unity."
Penn State fans certainly wanted Redd to stay.
Most downtown businesses displayed "Proud to Support Penn State Football" signs on windows. Some stores started selling shirts with the slogan "Billeave," playing off of the first name of O'Brien, the former New England Patriots offensive coordinator.
Penn State fans and students also started a Twitter campaign to sway Redd with the hashtag "StaySilas."
"With Silas Redd, or without Silas Redd, I would not discount coach O'Brien's ability to make an offense and put points on the board," said Tim Sweeney, a businessman who hosts an online radio show and heads the Penn State football Lettermen's Club, a group of former players.
Information from ESPN.com's NoleNation's Corey Dowlar and The Associated Press was used in this report.