According to Mousavi, three other defendants in the case were sentenced to five, 15 and 25 years in prison for being members of IS.
In November, Tehran said that 26 “Takfiri terrorists” from Afghanistan, Azerbaijan and Tajikistan had been arrested in connection with the mass shootings.
In Shia-majority Iran, the term takfiri generally refers to jihadists or supporters of radical Sunni Islam.
The Shah Cheragh mausoleum is home to the tomb of Ahmad, the brother of the eighth Shia Imam Imam Reza and is considered the holiest site in southern Iran.
Last year’s shootings came as nationwide protests erupted in Iran after the custodial death of 22-year-old Mahsa Amini, arrested for allegedly violating strict dress rules for women.