Yesterday's election was widely seen as a referendum on the 68-year-old cleric's push for greater freedom at home and outreach to the wider world, which culminated in the completion of a landmark 2015 nuclear deal that hard-liners initially opposed.
The nuclear deal won Iran relief from international sanctions in exchange for limits on its contested nuclear program. But Iran continues to suffer from high unemployment and a dearth of foreign investment, putting pressure on Rouhani to show he can do more to turn the sluggish economy around.
"Today, Iran, prouder than ever, is ready to promote its relations with the world based on mutual respect and national interests," he said in a televised addressed flanked by photos of Iran's supreme leader, Ayatollah Ali Khamenei, and his predecessor, the late Ayatollah Ruhollah Khomeini, the leader of Iran's 1979 revolution.
Iran "is not ready to accept humiliation and threat," he continued. "This is the most important message that our nation expects to be heard by all particularly world powers."
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Although considered a moderate by Iranian standards, Rouhani was the favorite pick for those seeking more liberal reforms in the conservative Islamic Republic.
He appeared to embrace a more reform-minded role during the campaign as he openly criticised hard-liners and Iran's powerful Revolutionary Guard, a paramilitary force involved in the war in Syria and the fight against the Islamic State group in neighbouring Iraq.
Many female drivers held out the V for victory sign and flashed their car lights on highways to celebrate the win in Tehran's affluent north.
"I feel that I did a huge thing. I voted for my country's future," said one, 32-year-old Sarah Hassanpour, who wore a loosely fitting headscarf covering only the back of her head. "I am so happy, because there will be no war and insecurity." Rouhani was first elected in 2013 with nearly 51 percent of the vote.
"Though he'll remain a centrist, Rouhani will be more aggressive in pursuing reforms," he predicted, though he cautioned it will not be easy.
"Rouhani will continue to face an uphill climb on political reform; the hard liners will dig in around education and other issues," he wrote. "But working with a centrist parliament, he will begin to ease the political darkness that followed the 2009 election."
Iran's president is the second-most powerful figure within Iran's political system. He is subordinate to the supreme leader, who is chosen by a clerical panel and has the ultimate say over all matters of state.