The 6-foot-9, 277-pound Villanueva, who is currently a member of the U.S. Army Rangers reserve unit, played offensive line, defensive line and wide receiver at Army. In 2009, he caught 34 passes for 522 yards and five touchdowns.
The Steelers plan to use Villanueva on the offensive line.
Villanueva has served three tours in Afghanistan and was given the Bronze Star Medal for valor, among other honors.
He signed with the Eagles in May but was cut in August.
Pittsburgh opens the season Sept. 7 against the Cleveland Browns.
- Chris Johnson