Whitford will appear as a gay man named Abraham, described as "politically active" and "sophisticated", according to TV Guide.
Shameless stars William H Macy as the single father of six children who learn to take care of themselves.
Whitford is best known for his characters on The West Wing and Studio 60 on the Sunset Strip.
The actor previously praised The West Wing, calling it "the most amazing thing" he's seen in show business.
"As a creative experience, it was beyond my highest expectations and for it to be in an arena that, at that particular time and that I've always felt and that I continue to feel is an urgent, fascinating arena... it was a really good gig," he said.
Season three of Shameless premieres on Sunday, January 13 at 9pm on Showtime.
Watch a clip of Shameless below: