The rumours of 75-year-old returning as Professor Xavier in the final spinoff were doing the rounds since quite some time.
"Worst kept secret. #ProfessorX #Wolverine3," Stewart tweeted.
Producer Hutch Parker also confirmed Stewart's appearance, reported Digital Spy.
"I understand that Professor Xavier will be making more than an appearance, however, and that intrigues me," he said.
The final outing for Wolverine is said to be inspired by Mark Millar's "Old Man Logan" series, and may have a Western tone.
Stephen Merchant, Richard E Grant and Boyd Holbrook are also set to star.