The former MVP and four-time Pro Bowl selection spent two seasons with Baltimore after playing 11 with the Houston Oilers/Tennessee Titans franchise.
He was injured most of the 2007 season when he played in only six games.
"I enjoyed it ... I look back and wouldn't change a thing," he said at a news conference at Ravens headquarters in Owings Mills, Md. "It was a difficult decision. In my mind, I felt like I could still play but physically I just can't do it anymore."
Overall, he completed 60.1 percent of his passes for 31,304 yards, 174 touchdowns and 119 interceptions in 161 regular-season games.
The Oilers made him a No. 1 pick in the 1995 NFL draft out of Alcorn State.