Jundullah (Soldiers of God), which says it is fighting for the culture and faith of the ethnic Baluch people, claimed responsibility for the attack in a statement on its website.
A bomb explosion in the south-eastern Iranian city of Chabahar has killed at least 39 people, the official Iranian news agency says.
The suicide attack took place outside the Imam Hossein Mosque, Irna reported.
The militant Sunni Muslim group Jundullah claimed responsibility for the bombing.
The province of Sistan-Baluchistan is prone to unrest, with the mainly Sunni population often claiming discrimination by Shias.