The run-off vote is being held a week after none of the candidates were able to get an outright victory during the first round of the snap election.
Iran’s Supreme Leader Ayatollah Ali Khamenei cast his ballot when the polls opened at 8 am local time (0430 GMT), state media IRNA reported.
“God willing, our dear people will succeed in voting and choosing the best. At this stage, people should naturally be more determined because they are completing the task,” Khamenei told reporters in Tehran.
“Tomorrow, God willing, we will have our president,” he added.
Over 60 million Iranians are eligible to cast their ballots to elect a new president.
"We are starting the second round of the 14th presidential election to choose the future president from among the two candidates across 58,638 polling stations in the country and all stations abroad," said Iran’s interior minister, Ahmed Vahidi, according to Iranian state TV.
In last week’s first round, Masoud Pezeshkian, the lone reformist candidate, won 10.41 million votes, while his ultraconservative rival, Saeed Jalili garnered 9.47 million out of 24.5 million votes counted, the electoral body said, reported the state-owned IRNA news agency.
Pezeshkian and Jalili are contesting in the run-off to become the new president.
The results showed that the turnout was about 40 percent — the lowest ever recorded since the establishment of the Islamic Republic in 1979.
On Wednesday, Khamenei called for a higher turnout in the run-off election.
Mohammad Bagher Ghalibaf, the speaker of the Iranian parliament, who was also running for the position, won 3.38 million votes in the first round.
Ghalibaf called on his supporters to vote for Jalili in the run-off following his defeat.
The election went to a run-off after none of the candidates were able to gain 50 percent plus. Any candidate who wins the highest number of votes in the second round on Friday will be declared the winner of the race.
The vote was originally scheduled for 2025, but it was brought forward after former president Raisi died in May in a helicopter crash alongside several companions including late Foreign Minister Hossein Amir-Abdollahian.
