The Iranian team, who had earlier lost to Bulgaria and Canada in Poll G, was defeated against USA in straight sets (25-23, 26-24, 26-24) in Sofia.
Benjamin Patch had his best scoring performance of the championship with 17 points. Jake Langlois added 10 for USA. Milad Ebadipour was the only Iranian in double figures finishing with 10.
The 2018 championship is the 19th staging of the FIVB Volleyball Men's World Championship, contested by the senior men's national teams of the members of the Fédération Internationale de Volleyball (FIVB), the sport's global governing body.
The final tournament is being held in Italy and Bulgaria from September 9 to 30. For the first time the tournament is jointly-hosted by more than one country.
The final six will be hosted by Italy at the Pala Alpitour in Turin