Police were sent to Young's apartment to check on him after he missed work on the series Monday morning. He was found dead from a self-inflicted gunshot wound.
Young's manager Jonathan Baruch issued a statement confirming his death Monday.
"It is with great sadness that I announce that Lee Thompson Young tragically took his own life this morning," Baruch said. "Lee was more than just a brilliant young actor, he was a wonderful and gentle soul who will be truly missed. We ask that you please respect the privacy of his family and friends at this very difficult time."
Young, 29, played Boston Police detective Barry Frost, partner to Angie Harmon's Jane Rizzoli, in the police drama.
TNT had announced Friday that the show had been renewed for its fifth season on TNT. New episodes in the fourth season will air until September 10, when the show will take a hiatus until spring of 2014.
There are a few episodes with Young still set to air. Young's most memorable roles were in "The Famous Jett Jackson," "Smallville," Scrubs" and the movie "Friday Night Lights."