Polling stations opened at 8 a.m. local time (0430 GMT) on Friday and are scheduled to close at 6:00 p.m. (1430 GMT), with the possibility of extensions by the Interior Ministry.
Leader of the Islamic Revolution Ayatollah Seyed Ali Khamenei cast his ballot at a polling station in Tehran just as the voting began.
"Praise God, it is a good day. It’s a day where the people are present and are participating. It’s a day where our dear people are actively involved in an important political matter of the country, that is, the elections. I’ve heard that people’s enthusiasm and interest are greater than before. I hope that is the case. If that is the case, it is something to be pleased about. God willing, our dear people will be able to vote and choose the best [candidate]. At this stage, people should naturally make a greater effort to finish the task so that tomorrow, God willing, we will have our president. God willing, may God grant success to our nation, help our country to develop, and shower His grace and mercy upon all those who are working hard on this path," the Leader said after casting his vote.
The runoff follows the June 28 election in which Masoud Pezeshkian and Saeed Jalili emerged as frontrunners, yet neither secured an outright majority.
More than 24 million people participated in the June 28 election.
The election seeks to appoint a successor to President Raeisi, who passed away on May 19 in a tragic helicopter crash.