"I have decided to retire from the NFL," Thomas wrote Sunday morning on his Twitter page.
Thomas sent a subsequent message on Twitter in what appeared to be an explanation for his decision to retire.
"God has blessed me (with) achieving a childhood dream," Thomas wrote. "I want to give back to my hometown and coach/mentor kids so they can reach their own."
Thomas told ESPNChicago.com that he'd been considering retiring for some time, but the daily grind of training camp played a role in coaxing out the decision.
"I definitely feel like it's a weight lifted, the beginning of my true life. I'm very excited to be able to live my life without anything holding me back.
"It's something I meditated on for some time. But as I was preparing for the upcoming season, my spirit was telling me it was time to turn over a new leaf and put away this childish game idolized by so many people."
Bears coach Lovie Smith said he didn't attempt to convince Thomas to stay.
"He's not with us anymore; decided he didn't want to play any more football," Smith said. "You don't want a guy out here if he's not totally into football and he has a passion to play it. So it's not like I try to talk guys out of it when they've made up their minds."
A second-round draft pick of the Washington Redskins in 2008, Thomas played in 55 career games (including 11 starts) with the Washington Redskins, Carolina Panthers and New York Giants, with whom he earned a Super Bowl ring in the 2011 season.
In four seasons in the NFL, Thomas, 25, caught 43 passes for 482 yards and three touchdowns, in addition to returning 60 kickoffs for 1,445 yards.
The Bears signed Thomas in March to a one-year contract.
The club expected Thomas to contribute mainly on special teams in 2012, but his departure might clear the way for younger players to earn roster spots such as second-year receiver Dane Sanzenbacher and undrafted rookie Brittan Golden.
Sanzenbacher appeared to face long odds of making the roster when the club brought in Thomas because of the new addition's abilities on special teams. Golden, meanwhile, seems to have caught the eye of the coaching staff after strong workouts early in training camp.
"It's understandable," Sanzenbacher said of Thomas' decision. "Everyone is at a different point in their career, has different things going on outside of football, different personal things. We don't know. We come here (and) it's like you and your co-workers. You're with them for that amount of time, and some people like to pursue other things."
Thomas sounded ready for his next phase.
"The younger me would never have thought to retire," Thomas said. "But as I've grown spiritually, it's allowed me to see a bigger purpose in life. It's not about the glitz and the glamour. It's about my life and making me happy, serving out my life's purpose, enjoying family and doing what I was called to do."