The photo on the stamp is based on a portrait of Stewart as he appeared in a publicity photo for 1949's "The Stratton Story."
Stewart, who died in 1997, starred in more than 80 movies, including "It's a Wonderful Life," "Rear Window" and "Mr. Smith Goes to Washington." He won an Oscar in 1941 for "The Philadelphia Story."
The Pennsylvania ceremony will be held at the Jimmy Stewart Museum in Indiana, the actor's hometown. The other will be held at Universal Studios in Hollywood.