Fey says she is quitting the show after six seasons as head writer and co-anchor of the "Weekend Update" fake news segment to focus on her new NBC prime-time series, "30 Rock," which debuts Oct. 11.
"I'm out of the fake news business now," Fey told the Television Critics Association's summer meeting Saturday.
She announced her "SNL" departure on Friday night's "The Tonight Show with Jay Leno."
"The new show's going to take a lot of time," Fey told Leno. "I wanted to stop doing 'Saturday Night Live' so I could spend more time with Star Jones," she joked, referring to last month's dramatic departure of the co-host of "The View."
"30 Rock" is a spoof of a late-night talk show, coincidentally "The Girlie Show." Fey is "is being pushed to the edge of reason by the show's two stars and a demanding network honcho," according to NBC's Website.
"This is the big leap I'm making, it's a show about working at a late-night comedy show," she told Leno. "I'm very creative."
Also leaving "SNL" is regular Rachel Dratch, who plays a sketch actress on Fey's new sitcom.
"If you're asking if I'm happy about it, no," said Lorne Michaels, executive producer of "30 Rock" and "SNL." "Tina disappearing is a huge hole in the writing staff."
Michaels said various individuals and duos would be tested in September to decide who will replace Fey on the "Weekend Update" segment.
Fey, 36, first joined "SNL" as a writer in 1997 and became head writer in 1999.
She is writing, starring and executive producer of her new show.
"I'm going to go nuts," she said Saturday. "I do not understand the train that is about to hit me. We're trying to get as much done in the writers' room now before we start and I tried to gather a staff of writers who are experienced so they can proceed when I'm on the set."
"30 Rock" also stars Alec Baldwin and former "SNL" regular Tracy Morgan.