Aref, a leader in Iran's reform movement, had been asked to withdraw by former president Mohammad Khatami, who had endorsed Hassan Rouhani, the only cleric in the race, The Washington Post reported.
Speaking at a rally Monday night, Aref urged all Iranians to vote in the election even though most reformist candidates had been excluded from running.
"To get the country back on the track of rationality, we must have a high turnout," he told several thousand supporters at a Tehran stadium. "Not voting is not a protest."
Six candidates are now running for president, including the mayor of Tehran, a former foreign minister and the country's lead nuclear negotiator.
Gholam Ali Haddad-Adel, a conservative and the former speaker of parliament, dropped out Monday.
Aref's popularity had increased in recent weeks after his strong performances in three televised debates. He has not endorsed any of the remaining candidates.