"I announce my intention to run in the 10th presidential election hoping to fulfil my duty," he said in a statement carried by the ISNA and Fars agencies.
A reputed moderate, Mousavi was prime minister from 1981 to 1989, under Ayatollah Ali Khamenei's presidency, who is now the Islamic republic's supreme leader.
Mousavi was the last to serve as premier, a post which was scrapped in Iran's revised constitution.
