Mohammad Mokhtari, Mehdi Shirmohammadi, Mostafa Kiani, Movahed Mohammadpour, Ali Mirshekari and Hassan Abdollahi scored for Iran.
Okuyama Mansouri scored Japan’s only goal.
Russia also defeated Paraguay 8-7.
Team Melli will meet Russia in Group B on Thursday.
Iran won the last Intercontinental Beach Soccer Cup in 2019 with a 6-3 win over Spain.