The writer will contribute a storyline to Robert Kirkman's Image Comics title, it was revealed at the Skybound imprint panel on New York Comic Con.
"I'm behind on everything else, but ahead on that," Diggle said.
"Stepping up to this, you guys have created these really strong characters. It was really easy to step into. Usually [taking over existing characters is] like pulling teeth with me."
Diggle was recently announced as the new writer on Action Comics following the departure of Grant Morrison.
He is also working on a untitled project for Dynamite Entertainment's new crime imprint.
Diggle will join Thief of Thieves from issue #14.