Williams rushed for 444 yards and two touchdowns last season, backing up Pro Bowl running back Ray Rice.
"Thank you all, but this ain't it, I'm gonna do something really special," Williams wrote on his Twitter account. "Be you and change the world."
Williams rushed for 10,009 career yards and 66 touchdowns, catching 342 passes for 2,606 yards and eight touchdowns.
He played in the NFL for 11 seasons, including stints with the New Orleans Saints and Miami Dolphins and was selected to the Pro Bowl in 2002 when he rushed for a career-high 1,853 yards and 16 touchdowns.
Williams played one season for Baltimore and is under contract through the 2012 season with the team.
This isn't the first time Williams would be retiring from football. The running back first retired in 2004 — one that would eventually only last a year before he returned to Miami for the 2005 season.
He was also suspended for the entire 2006 season after another failed drug test and instead spent the season playing for the CFL's Toronto Argonauts. He was the highest-paid running back in the CFL with his one-year, $240,000 contract.
Williams attended the University of Texas where he won the Heisman Trophy in 1998. He was the the fifth overall draft pick in 1999 by the Saints.
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