"This has been a very difficult decision. One of the toughest of my life. I have been torn back and forth," Sanchez said. "In the end, after sleepless nights and lots of prayer, I have decided that I will forego a fifth year at USC and declare my eligibility for the 2009 NFL draft.
"It has been my dream since I was a youngster to play in the NFL. Thanks to this great academic institution and football program I have been able to realize that dream.
"It is with a heavy heart that I say goodbye to this university, but I can't tell you how excited I am for this dream to come true."
Sanchez's decision came two weeks after his top performance as a college player.
The redshirt junior has seen his stock rise following his impressive performance against Penn State in the Rose Bowl, and is widely considered one of the top two quarterback prospects in the NFL draft, along with Georgia's Matthew Stafford.
Sanchez led the Trojans to a 12-1 record and Rose Bowl Championship in 2008 with his solid play and fiery leadership.
In his first year as a starter, he passed for 3,207 yards and 34 touchdowns while completing 65.8 percent of his pass attempts.
Sanchez's 34 touchdown passes are the second-most in a season by a USC quarterback behind Matt Leinart's 38 in 2003.
He is on pace to graduate from the Annenberg School of Communications this spring.
"It has been an absolute honor to represent this university and I will do my best to continue to do that, Sanchez said. " I hope that the Trojan Family will continue to support me as I take this next step."
Mark Sanchez was named the Offensive Player of the Game in USC's 38-24 Rose Bowl
win. (Getty Images/Harry How)