Thousands of Iranians have taken to the streets of Tehran, shouting pro-reform slogans and hailing Hassan Rouhani’s election as president.
The reformist-backed cleric won just over 50% of the vote and so avoided the need for a run-off.
Mr Rouhani said his win was a “victory of moderation over extremism”.
The US expressed concern at a “lack of transparency” and “censorship” but praised the Iranian people and said it was ready to work with Tehran.
Some 72.2% of the 50 million eligible voters cast ballots on Friday to choose the successor to Mahmoud Ahmadinejad.
Supreme Leader Ayatollah Ali Khamenei congratulated Mr Rouhani on his victory, saying: “I urge everyone to help the president-elect and his colleagues in the government, as he is the president of the whole nation.”
Ayatollah Khamenei will ratify the vote on 3 August and the new president will then take the oath in parliament.