Voters will choose between hard-line former nuclear negotiator Saeed Jalili and reformist candidate Masoud Pezeshkian, who has aligned himself with those seeking a return to the 2015 nuclear deal with world powers.
Domestic issues that have loomed over the race include a renewed crackdown on mandatory headscarfs for women and a proposed gasoline price hike, as well as years of economic malaise marked by widespread unemployment and high inflation.
After a record-low turnout in the first round of voting June 28, it remains unclear how many Iranians will take part in Friday's poll. Iranian law requires that a runoff if no one candidate gets more than 50% of all votes cast in the first round.
While 85-year-old Supreme Leader Ayatollah Ali Khamenei has final say on all matters of state, presidents can bend Iran toward confrontation or negotiations with the West.