The actor will reprise his character, Mike Ross, in at least one episode of the upcoming run, after bowing out of the drama after the seventh season, along with another original cast member, Meghan Markle, who quit to become a British royal.
"I couldn't be happier to be part of the final chapter of 'Suits'," he tells Deadline. "I've missed my TV family, and am looking forward to seeing what kind of trouble Mike Ross can stir up at the firm one last time.
"But if the wardrobe department thinks they're getting back the suits I stole when I left, they're going to be very disappointed."
Series creator and executive producer Aaron Korsh adds: "You can't contemplate it (final season) without contemplating bringing Mike back.... There was never a question in my mind that I wanted him back. He is the foundation of the show; it was built on him, others also but the very foundation we built on Patrick and Mike, and we all wanted to have him back.
"It was about how to integrate him organically and in a satisfying way. Then I had to make sure that he had the time to do it and could do it, he a new father also. After we came up with what it was going to be, I called him. I went to his house, he showed me his olive trees, which were awesome. We went out to dinner, and we talked about it. I said what we had in mind, he was very excited. And then we did it".
Meghan Markle, who is now the Duchess of Sussex, is not expected to be back for the final season of Suits. Her character, Rachel, wed Adams' Mike Ross at the end of season seven.
"If you take a look throughout season eight and nine, people refer to both Mike and Rachel... and when he comes back, I do believe there is a little update on Rachel," Korsh tells Deadline, but insists the drama won't mirror Markle's real life and her character won't be a new mom.
"We have not determined that they've (Rachel and Mike) had a baby," he adds.
Meanwhile, Adams has also signed on to portray astronaut John Glenn in the TV adaptation of Tom Wolfe's space race bestseller "The Right Stuff". The "Suits" star will lead the cast of National Geographic's scripted series as Glenn, the first American to orbit the earth.
Production on the new TV show will begin in Cocoa Beach, Florida, this autumn and the show will air in 2020.