Richard Nephew, a deputy to US special envoy for Iran Robert Malley, is no longer serving in that role but remains a State Department employee, it was reported on Monday.
A senior State Department official did not give a reason for Mr Nephew's change of role but told Reuters that personnel moves were "very common" a year into the administration.
The Wall Street Journal earlier reported that Mr Nephew left after differences of opinion in the US negotiating team on the Iran nuclear deal.
His appointment as deputy to Mr Malley, who was criticised for being too soft on Iran, had been an attempt to strike a balance on the team, with US Secretary of State Antony Blinken pushing for diversity of views on the Iran issue, US sources told The National.
This stance has delayed several appointments but Mr Nephew was an early pick for the job because of his expertise and close relationship with the Biden transition team.
Agencies contributed to this reporting