He faces stiff competition from hardline cleric Ebrahim Raisi, 56, who has positioned himself as a defender of the poor and called for a much tougher line with the West.
Supreme leader Ayatollah Ali Khamenei cast his ballot just minutes after polls opened at 8:00 am (0330 GMT).
"The destiny of the country is in the hand of Iranians," he told reporters as he voted in his compound in Tehran.
Rouhani, a 68-year-old moderate cleric, has sought to frame the election as a choice between greater civil liberties and "extremism".
He has pushed the boundaries over the past fortnight, criticising the continued arrest of reformist leaders and activists, and calling on security agencies not to interfere in the vote.
Raisi says he will stick by the 2015 nuclear deal with world powers, that saw curbs to Iran's atomic programme in exchange for sanctions relief, but he points to the continued economic slump as proof that Rouhani's diplomatic efforts have failed.
Rouhani responded by calling on voters to keep hardliners away from Iran's delicate diplomatic levers.
"One wrong decision by the president can mean war and a correct decision can mean peace," he said at his own Mashhad rally.
The election comes at a tense moment in US-Iran relations.
Rouhani gained a reprieve on Wednesday when the administration of US President Donald Trump agreed to continue waiving nuclear-related sanctions, keeping the deal on track for now.
"For me, Mr Rouhani's dialogue with the world and moderation in society are very important," said Zahra, a 32- year-old PhD student in food science.
Under former hardline president Mahmoud Ahmadinejad, "the sanctions really hurt us. It was hard to get lab equipment and very difficult to get visas to study abroad. Now my colleagues can travel to France and the US," she said.
Disclaimer: No Business Standard Journalist was involved in creation of this content
You’ve reached your limit of {{free_limit}} free articles this month.
Subscribe now for unlimited access.
Already subscribed? Log in
Subscribe to read the full story →
Smart Quarterly
₹900
3 Months
₹300/Month
Smart Essential
₹2,700
1 Year
₹225/Month
Super Saver
₹3,900
2 Years
₹162/Month
Renews automatically, cancel anytime
Here’s what’s included in our digital subscription plans
Exclusive premium stories online
Over 30 premium stories daily, handpicked by our editors


Complimentary Access to The New York Times
News, Games, Cooking, Audio, Wirecutter & The Athletic
Business Standard Epaper
Digital replica of our daily newspaper — with options to read, save, and share


Curated Newsletters
Insights on markets, finance, politics, tech, and more delivered to your inbox
Market Analysis & Investment Insights
In-depth market analysis & insights with access to The Smart Investor


Archives
Repository of articles and publications dating back to 1997
Ad-free Reading
Uninterrupted reading experience with no advertisements


Seamless Access Across All Devices
Access Business Standard across devices — mobile, tablet, or PC, via web or app
