Pileggi tweeted the news on Monday.
Very happy to announce that Walter Skinner will once again be getting all grumpy and bitchy with his two wayward kids. Very happy.
— Mitch Pileggi (@MitchPileggi1) April 20, 2015
The actor starred as Walter Skinner, Mulder and Scully's boss and friend, though occasionally an enemy, throughout "The X-Files'" nine-season run. He also appeared in both movies and the short-lived spin-off "The Lone Gunmen."
Pileggi had been rumored to return for weeks, and Duchovny teased his possible return during a recent interview with David Letterman.
Pileggi has most recently appeared on "Dallas," "Sons of Anarchy" and "Grey's Anatomy."