The brassy, outspoken Dickinson, who called her first book "Everything About Me Is Fake and I'm Perfect," and "Top Model" host Tyra Banks (search) have had their behind-the-scenes differences in the show's first four seasons.
Apparently, Banks wanted a change.
According to UPN's announcement, Dickinson is "leaving to pursue other projects," including a TV production deal and a new book. She's already wrapped her season of VH1's "The Surreal Life (search)."
"We would like to thank Janice for bringing her talents to the show and for everything she did to help us build 'America's Top Model' into its current success," Ken Mok, the show's executive producer, said in a statement.
"We wish her well in all of her future endeavors."
Dickinson was viewed as "Top Model's" harshest judge, often leveling severe criticism at the show's contestants vying for the title.
The British-born Twiggy shot to fame in "Swinging '60s" London and set the standard for wafer-thin supermodels.
She'll join Banks, fashion photographer Nigel Barker and runway expert J. Alexander as judges for season 5 of "Top Model" (Alexander was a "frequent special guest" in previous seasons).
"The show and participants will benefit a great deal from her vast knowledge and expertise," Mok said of Twiggy.
The next installment of "America's Next Top Model" returns in September — the same time that Banks will launch her new daytime talk show produced by Telepictures.