Polling stations opened at 8 am on Friday morning and were scheduled to close after 10 hours.
But the higher-than-expected turnout in the elections was enough to convince the Interior Ministry to extend the voting hours until 8 pm all around the country.
Over 61 million Iranians are eligible to cast their ballots at around 59,000 polling stations across the country.
There are four candidates seeking presidency in the polls, including Saeed Jalili, Mostafa Pourmohammadi, Mohammad Baqer Qalibaf, and Masoud Pezeshkian. Two other hopefuls, Alireza Zakani and Amir Hossein Qazizadeh Hashemi dropped out of the race on Wednesday and Thursday.
The polls take place a year ahead of schedule, as Iranian President Ebrahim Raisi passed away in May.
A helicopter carrying President Raisi and his entourage crashed in northwestern mountainous forests on May 19, killing the president, Foreign Minister Hossein Amirabdollahian, and six others.