Collins, the Titans' off-and-on starter since 2006, released a statement in which he said after several months of "introspection," he decided that "my willingness to commit to the preparation necessary to play another season has waned to a level that I feel is no longer adequate to meet the demands of the position."
That sounded much different than comments Collins made to The Tennessean in June, when he said: "I know I still have good football left in me, and I want to see where (the Titans are) at, too. If it makes sense on both sides to do it, I'd be good for coming back."
But Collins isn't coming back, so the Titans are left with first-round draft pick Locker and second-year pro Rusty Smith at quarterback. The team said earlier this year that it would trade or release Vince Young, its 2006 first-round pick, after a tumultuous time together.
"I want to wish Mike Munchak and the Tennessee Titans the best of luck in the future," Collins said Thursday in his statement. "I have had a fantastic five years here, and my family and I look forward to remaining part of the Middle Tennessee community that has been extremely gracious towards us and an honor to play for."
Collins, 38, enjoyed his best season with the Titans in 2008, when he took over for an injured Young and started 15 games, going 12-3 as the team won the AFC South. Collins then signed a two-year, $15 million contract that expired after the 2010 season.
Collins shared the stage with Young last season despite suffering injuries to his middle finger and thumb. Collins took over for good after Young suffered a season-ending injury to his right thumb Nov. 21, though the Titans only won one of their final six games in finishing 6-10.
"Kerry had a great NFL career, and we have been very fortunate to have had him with our organization for the last five years," Titans coach Mike Munchak said in a statement released by the team. "I really enjoyed working with him. He was a true pro who loved the game. His work ethic and commitment were a great example to his teammates of what it took to be successful in this league."
Said Titans general manager Mike Reinfeldt in a statement: "Kerry accomplished a great deal in his career both personally and as the leader of some great teams. He was a talented passer and became a true professional. We were fortunate to have him as a member of our team over the last five years, and we wish him and his family the best."
Collins threw for 40,441 yards -- good for 11th on the NFL's all-time list -- with 206 touchdowns and 195 interceptions in 195 career games. He was the fifth overall pick by the Carolina Panthers in the 1995 NFL Draft and also played for the New Orleans Saints (1998), New York Giants (1999-2003) and Oakland Raiders (2004-05) before joining the Titans.
As a starter, Collins lead his teams to the playoffs four times, including an NFL Championship Game appearance during the 1996 season with the Panthers and a Super Bowl appearance with the Giants in the 2000 season.
The Associated Press contributed to this report.