The populist former leader's registration puts pressure on Supreme Leader Ayatollah Ali Khamenei. In office, Ahmadinejad openly challenged the 85-year-old cleric, and his attempt to run in 2021 was barred by authorities.
The firebrand, Holocaust-questioning politician's return comes at a time of heightened tensions between Iran and the West over Tehran's rapidly advancing nuclear program, its arming of Russia in its war on Ukraine and its wide-reaching crackdowns on dissent.
Associated Press journalists in Tehran saw Ahmadinejad arrive at the Interior Ministry and begin the registration process on Sunday. Before his arrival, his supporters chanted and waved Iranian flags.
An election is planned June 28 to replace Khamenei's hardline protege Raisi, who died in a helicopter crash in May along with seven other people.
Ahmadinejad previously served two four-year terms from 2005 to 2013.
Under Iranian law, he became eligible to run again after four years out of office, but he remains a polarising figure even among fellow hardliners. His disputed re-election in 2009 sparked massive "Green Movement" protests and a sweeping crackdown in which thousands of people were detained and dozens were killed.
Abroad, he became a caricature of Western perceptions of the Islamic Republic's worst attribute, questioning the Holocaust, insisting Iran had no gay or lesbian citizens and hinting Iran could build a nuclear weapon if it chose to do so.
But Ahmadinejad remains popular among the poor for his populist efforts and home-building programs. Since leaving office, he's raised his profile via social media and written widely publicised letters to world leaders.
He's also criticised government corruption, though his own administration faced graft allegations and two of his former vice presidents were jailed.
Khamenei warned Ahmadinejad in 2017 that his standing for office again would be a "polarised situation" that would be "harmful for the county."
Khamenei said nothing during Ahmadinejad's 2021 attempt, when his candidacy was rejected by the 12-member Guardian Council, a panel of clerics and jurists ultimately overseen by Khamenei. That panel has never accepted a woman or anyone calling for radical change to Iran's governance.
That panel could reject Ahmadinejad again. However, the race to replace Raisi has yet to draw a candidate with clear, overwhelming support from Khamenei.
