The girl was murdered last February in the southwestern Iranian province of Khuzestan.
The murder caused an outcry in Iran and the rest of the world, not least when Iranian media reported that the girl had been married off at the age of twelve.
When she was killed, she had a three-year-old son with her killer.
According to media reports, the man was her cousin.
An additional person is sentenced
Another person is sentenced to nearly four years in prison for aiding and abetting.
According to Iranian law, Iranian girls can be married off at the age of 13.
For boys, the limit is 15. However, marriages can take place earlier than that, according to the UN agency OHCHR, with the consent of parents and permission from judges.