The actor will be leaving by mid-season, and not on "sabbatical" like he did in 2006, the season's 10th episode will be Petersen's last.
The show's executive producer Carol Mendelsohn confirmed the rumors.
"Billy is leaving," she said. "But he will remain throughout the run of the series an executive producer. And he will, whenever CBS asks, come back. I don't think you've seen the last of Gil Grissom."
Petersen, in a July 11th interview, was hesitant to state that is departure is permanent citing his continuing role as executive producer.
"I want it to work for the writers, I want it to work for the cast, and, most importantly, I want it to work for the audience," he said. "I don't want them to abandon the show."
On the bright side Grissom's love interest and former CSI agent Sara Sidle, played by Jorja Fox, will be returning to the show starting with the season premier.
Petersen said that his character's departure is due to a personal crisis set into motion by Dourdan's character's death. Sidle returns to help her man through the tough time.
"The easiest way to describe Grissom is 'in crisis,'" Petersen said. "As a man. As a scientist. As a teacher. As a middle-aged person who has been very successful at what he's done... [he] wonders, 'What's the point?'"Although he doesn't reveal what transpires, Petersen says that Fox's character will be a big part of the picture.
"I wouldn't want to say exactly what we're going do -- I want people to watch, certainly," he chuckled. "But Sara is involved... It's often darkest just before the dawn."
CSI: is currently the CBS network's top rated show. Petersen's departure will be the third major character's in the past year.
The news comes in a timely manner. It is shark week, so jump Grissom, jump!
