Lopez broke the news to "Idol" host Ryan Seacrest on his radio show.
"I really was dreading this phone call with you," she told him. "I honestly feel like the time has come that I have to get back to doing the other things that I do that I've put kind of on hold because I love 'Idol' so much."
"American Idol" has been TV's most popular program for nearly a decade, but its ratings have declined as TV's talent show field has grown more competitive.
When it returns for its 12th season next January, "Idol" will have to be a reengineered version of the once-powerhouse series that turned hopefuls such as Jennifer Hudson into stars and gave network rivals fits.
Speculation about potential new panelists has focused on Mariah Carey, with former "Idol" runner-up Adam Lambert getting a shout-out from his fans. Much is at stake for the show and for its judges: Lopez's contract reportedly was worth $20 million and the stint proved a career-booster for her.
Fox and Jackson did not immediately respond to requests for comment.
Tyler started the exodus with his announcement Thursday that he was putting rock 'n' roll ahead of the show that he said had been "over-the-top fun."
"I strayed from my first love, Aerosmith, and I'm back — but instead of begging on my hands and knees, I got two fists in the air and I'm kicking the door open with my band," Tyler said in a statement. Aerosmith is on a nationwide tour with Tyler and has an album due out in the fall.
In a phone interview Thursday with The Associated Press as the news about Tyler broke, Lopez had said she was saddened to hear that Tyler was leaving and that his departure would play a role in her decision.
"I can't even imagine anyone else there right now because I've just spent two years sitting next to him," she said. "I love Steven, and we became close during that time. We were a great support for each other, on an adventure that neither one of us knew what it was going to be. So it's hard to hear that he won't be doing it."
Jackson and his "dawg!" exclamations have been a stalwart part of "Idol." But the show's multi-year contracts, such as the one Jackson signed, typically include an escape clause that the network can choose to exercise or not.
In a statement Thursday, one Fox executive expressed measured regret over Tyler's decision.
"We are very sad that Steven has chosen to focus more on his music, but we always knew when we hired a rock 'n' roll legend, he would go back to his music," said Mike Darnell, the network's president of alternative entertainment.
Seacrest, the other original cast member, will provide a stable center. He signed a two-year contract with "American Idol" producers this spring to continue as host.
The Fox series, which began with Jackson, Simon Cowell and Paula Abdul as judges, has made changes on the panel over the years. Ellen DeGeneres joined one season, and songwriter Kara DioGuardi was added as a fourth judge to work with Jackson, Cowell and Abdul.
The addition of Tyler and Lopez two years ago added fresh star power, and the pair and Jackson clicked on-camera.
But "American Idol" ended up in a vulnerable position, coming off a May finale that drew 21.5 million viewers as Phillip Phillips was crowned the newest winner.
It was a record low finish and followed a pattern of declining viewership for the once-inevitably top-rated series, not to mention a pattern of "white guys with guitars," as fans describe the show's recent string of similar winners.
The series ended the 2011-12 season as No. 2 with an average 20 million viewers for the Wednesday performance episodes, just edging third-place "CSI" on CBS (19.7 million) and NBC's top-rated "Sunday Night Football" (20.7 million).
"Idol" still rules among the most-watched talent shows, topping ABC's "Dancing With the Stars" and NBC's "The Voice." But the Fox show is fighting to keep advertiser-favored young viewers, with "The Voice" nearly equaling its audience among adults 18 to 49.
Fox, which from the start has protected "Idol" by programming it just once a year, has now diluted its uniqueness with a family competitor, Cowell's "The X Factor," which debuted last fall. After a disappointing start, the show underwent its own judges' shakeup to add Demi Lovato and Britney Spears.
The winner, however, ultimately is up to the audience. The judges are in the network's and producers' control, and their next moves are critical ones.
Copyright 2012 The Associated Press.