Zellner spent the first four years of his career with the Cowboys, starting 17 games the last two seasons. The Redskins previously signed defensive tackle Brandon Noble, a starting defensive tackle with Dallas last year.
The 6-5, 262-pound Zellner never turned into a pass-rushing force for the Cowboys. His best season was in 2001 when he replaced an injured Ebenezer Ekuban at right end and had 51 tackles and three sacks. Last year, Zellner started just two of 16 games and had 42 tackles.