Iran lost to the defending champion 65-41 on Day 3 of the Games at the Ariake Arena.
Iran’s Mohsen Sayyari topscored 17 points and Steve Serio scored 12 points for USA.
The competition began on August 25 and will continue until September 5 at two venues in Tokyo: Musashino Forest Sports Plaza for the group matches and Ariake Arena for the finals.
Germany, Algeria, Britain and USA are also in Group B. The top four of the six teams in each group go through to quarters.