Iran's Foreign Minister Mohammad Javad Zarif has said he is stepping down, in a surprise announcement posted to his Instagram account.
He apologised for "shortcomings" during his time in government.
Mr Zarif played a prominent role in negotiating the landmark 2015 nuclear agreement between Iran and major international powers.
But the future of the deal has been put into doubt after US President Donald Trump ended US involvement.
Mr Zarif's resignation was confirmed by Iran's state-run news agency, IRNA, which cited a spokesman for the foreign ministry.
The 59-year-old was educated in the US and holds a PhD in international law from the University of Denver, says a BBC report.
He has served as Iran's ambassador to the UN and became foreign minister in 2013 after President Hassan Rouhani was elected promising a more moderate, outward-looking Iran.