Shearer -- whose characters include Mr. Burns, Ned Flanders and Seymour Skinner -- quoted a lawyer for "Simpsons" co-creator James L. Brooks as saying he "would not be a part" of the show going forward.
In a follow-up tweet, Shearer explained that he "wanted what we've always had: the freedom to do other work."
Executive prouder Al Jean confirmed the news on Thursday, adding that the show - which was renewed last week for a 27th and 28th season -- will go on.
According to Entertainment Weekly, while other principal cast members Dan Castellaneta, Julie Kavner, Nancy Cartwright, Yeardley Smith and Hank Azaria all signed contracts to continue with the show for two more seasons, but Shearer had not. To make sure there was no delay in the airing schedule, the show began production on season 27 without him, though producers remained hopeful that he'd ultimately sign on.
Shearer, 71, thanked fans for their support. He's got no plans to retire, he added, and is moving "on to new stuff."
"The Simpsons" finishes up its 26th season this Sunday on Fox.