Hanson pitched five seasons in the major leagues, four with the Braves and one with the Los Angeles Angels in 2013 following a trade. He was 29.
Braves president John Schuerholz released a statement Tuesday morning.
"We are incredibly saddened to learn of Tommy's tragic passing," Schuerholz said. "He was a favorite in our clubhouse and with our staff and he will truly be missed by everyone in Braves Country. Our thoughts and prayers are with his wife, family, former teammates and friends."
A group of Hanson's former teammates was nearby when he died at Piedmont Hospital in Atlanta, a Braves representative told MLB.com.
Teammates and other players took to Twitter to grieve the loss:
Hanson had been in a coma, according to multiple reports.
A source told WSB-TV, the ABC affiliate in Atlanta, that Hanson had trouble breathing early Sunday morning and was taken to a hospital. He suffered catastrophic organ failure, multiple sources told the TV station.
Hanson was selected by the Braves in the 22nd round of the 2005 draft and was once considered the top prospect in the organization.
However, after posting a 2.89 ERA over 21 starts in his rookie season of 2009, Hanson slipped each season thereafter. He saw a gradual decline in his velocity and dealt with shoulder issues that kept him on the disabled list in 2011.
He was 49-35 with a 3.80 ERA for his career.
Information from ESPN.com's Doug Padilla was used in this report.