Paula Abdul has been with the show since it began
American Idol judge Paula Abdul is quitting the TV singing contest after eight years.
Making the announcement on Twitter, the 47-year-old said: "With sadness in my heart, I've decided not to return."
She did not give any reasons for her decision which ends weeks of speculation about contract talks and her future on the show
American Idol host Ryan Seacrest said on his Twitter page that he was "shocked and saddened" by the news.
"Idol and the cast have grown with each other over the last eight seasons," he added.
"I can't imagine the panel without Paula - she's a star and a great friend."
Abdul, meanwhile, wrote that she would "miss nurturing all the new talent".
"But, most of all, being a part of a show that I helped from day one become an international phenomenon," she added.
"What I want to say most is how much I appreciate the undying support and enormous love that you have showered upon me."
On Tuesday, it was announced songwriter Kara DioGuardi would be returning to the show to judge along with Simon Cowell and Randy Jackson.
TV network Fox said it was "saddened" that Abdul would not be returning to the show, saying she had been "an important part of the American Idol family over the last eight seasons".
"While Paula will not be continuing with us, she's a tremendous talent and we wish her the best."
American Idol remains the most popular show in the US
Abdul's future with American Idol had been unclear since the last series ended in May.
Speculation increased further as producers agreed new contracts with some of the other on-screen talent.
Last month, Seacrest reportedly signed a $15m (£8.9m) deal to front American Idol for the next three years.
In an interview last month, Abdul said she had been invited to stay with the show as long as it aired and that she was optimistic about negotiating a new contract.
Despite a decline in ratings, American Idol remains the most popular show in the US with the last series drawing an average of 26.3 million viewers.
It has spawned stars including Kelly Clarkson, Carrie Underwood, Jordin Sparks and Chris Daughtry.