Set scores were 25-18, 22-25, 25-18 and 25-12 at Hiroshima's Green Arena.
Aaron Russell led USA with match-high 18 points and Amir Ghfour had 17 points for Iran.
Iran national volleyball team is scheduled to play Tunisia on Thursday.
The 2019 edition of the FIVB Volleyball Men's World Cup features 12 national teams from the five continental confederations playing in Nagano, Fukuoka and Hiroshima in a round-robin format from October 1 to 15.
The Men's World Cup kicked off for five match days in Fukuoka and Nagano and the teams have moved to Hiroshima for the remaining six match days.