The find could boost the country’s proven reserves by a third as it struggles to sell energy abroad amid US sanctions.
Mr Rouhani made the announcement in a speech in the desert city of Yazd.
He said the field was located in Iran’s southern Khuzestan province, home to its crucial oil industry.
He added that 50 billion barrels would be added to Iran’s proven reserves of some 150 billion.
Proven reserves are those that are commercially feasible to extract.
The new oil field could become Iran’s second-largest field after one containing 65 billion barrels in Ahvaz.
“I am telling the White House that in the days when you sanctioned the sale of Iranian oil, the country’s workers and engineers were able to discover 53 billion barrels of oil,” Mr Rouhani said.
Iran currently has the world’s fourth-largest proven deposits of crude oil and the world’s second-largest deposits of natural gas.
Since the US withdrew from the 2015 nuclear deal, the other countries involved – Britain, Germany, France, Russia and China – have been struggling to save it.
However, they have offered no means by which Iran can sell its oil abroad.
The collapse of the nuclear deal coincided with a tense summer of mysterious attacks on oil tankers and Saudi oil facilities that the US blamed on Iran.
Tehran denied the allegations, though it did seize oil tankers and shoot down a US military surveillance drone.
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