Mr Mousavi was Iran's prime minister when the post was abolished in 1989
Two leading reformist challengers to Iranian President Mahmoud Ahmadinejad have registered to run against him in 12 June presidential election.
Former PM Mir-Hossein Mousavi, backed by former President Mohammad Khatami, is seen as the leading challenger.
Former parliamentary speaker Mehdi Karroubi has also put down his name - a day before the deadline expires.
They join President Ahmadinejad and the former Revolutionary Guards chief, Mohsen Rezai, in the race so far.
Mr Ahmadinejad is hoping to win a second term in office.
Although he has not spelled out his plans, he said that "serving the Iranian nation is the biggest honour".
'Standing for change'
"I have come to establish better ties between Iran and the world by removing tension and through constructive interaction," said Mr Mousavi after submitting his bid to the interior ministry.
Mr Mousavi, who was Iran's prime minister in the 1980s before the post was abolished, has promised to create new jobs and fight Iran's "extremist" image abroad, but also to pursue its controversial nuclear programme.
Mr Karroubi, the other reformist challenger, is one of the few Iranian politicians who has criticised Mr Ahmadinejad over his dismissal of the Holocaust as a "myth".
Describing the current regime as "incapable and unfit" to run the country, the 72-year-old veteran politician said he was "standing for change".
Mr Karroubi - who lost to Mr Ahmadinejad in the 2005 election - also demanded that the Revolutionary Guards "not interfere" in this year's election.
He had blamed his previous defeat on "illegal interference" by the Revolutionary Guards and their Basij militia.
"We want a free election without the interference of armed forces. We are ready to be defeated or be triumphant, but that should be according to the people's mandate," he said.
Analysts say reformers are hoping Mr Rezai's candidacy will split the conservative vote, but they warn that reformers, too, must be wary of splitting their potential support.