Some 63,500 polling stations opened at 08:00 local time (03:30 GMT) across the country and are to close at 18:00 (13:30 GMT), but voting hours will likely be extended amid estimates of a high turnout.
Expatriates can also cast their ballots in the presidential election at 310 polling stations in 102 countries.
Leader of the Islamic Revolution Ayatollah Khamenei was among the first to cast his ballot.
Besides picking a president, Iranians inside the country are also voting to choose members of the country’s City and Village Councils.
This year’s presidential race features four presidential candidates, namely incumbent Hassan Rouhani, who is running for a second term, Seyyed Ebrahim Raeisi, former deputy judiciary chief, member of Iran’s Expediency Council Mostafa Aqa-Mirsalim and former vice president, Mostafa Hashemi-Taba.
However, Rouhani and Raeisi are viewed as the viable contenders after two candidates from a field of six, Tehran Mayor Mohammad Baqer Qalibaf and Vice President Is’haq Jahangiri, dropped out in the final days of campaigning.
Rouhani has been campaigning on the platform of an active foreign policy meant to enhance international relations.
Raeisi has also vowed a strong economic management towards the elimination of poverty and unemployment.
Based on Iran’s election law, if no candidate manages to secure over 50 percent of the votes, a runoff will take place a week later.
There are more than 56 million eligible voters in Iran.
Weeks of heated campaigning came to an end on Thursday morning, 24 hours before the start of the elections.
Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei has called on Iranians to turn out en masse, saying the nation and Islamic establishment are the true winners of the vote no matter which candidate emerges victorious.