The Hollywood Reporter confirmed the 45-year-old model and 65-year-old fashion consultant will not return when Project Runway returns to Bravo for its 17th season.
Klum and Gunn will instead develop, produce and star in a new fashion reality show for Amazon Prime Video. Details are being kept under wraps, although Deadline said the series aims "to make fashion accessible to a wide audience."
"After 16 incredible seasons, I am saying 'Auf Wiedersehen' to Project Runway, a show that I was honored to host and help create," Klum said. "I am incredible proud of the show, and it will always have a special place in my heart."
"I am grateful to Project Runway for putting me on a path I never, in my wildest dreams, thought my career would take me!" Gunn added. "I am so proud to have been a part of the groundbreaking process that showcased talented young designers as never before."
Project Runway aired for five seasons on Bravo before moving to Lifetime for Seasons 6 to 16. Bravo reacquired the show this year after Lifetime's deal with The Weinstein Co. fell apart in the wake of Harvey Weinstein's sex abuse scandal.
Klum and Gunn have yet to address their exit on social media. The pair appeared in a photo together during an outing in Los Angeles last week.
"Look who I'm with in LA! The fabulous @heidiklum XOXO," Gunn wrote on Twitter.