The Obama administration's lead nuclear negotiator is leaving her post this summer.
State Department officials said on May 27 that Undersecretary Wendy Sherman will leave when nuclear talks between Iran and six world powers are complete.
Negotiators are hoping to clinch a final deal by June 30.
Sherman has led much of the technical discussions in Europe with Iranian officials over the last two years.
During her tenure the United States and Iran reached an interim agreement in November 2013 and a framework pact in April.
Secretary of State John Kerry, who will begin an intensive final round of negotiations with his Iranian counterpart on May 30, called his third-in-command a "critical" member of his team, and hailed her "tenacity and skill."
"It's been two long years," Sherman told The New York Times, saying the complex process of putting together an agreement was like solving a Rubik's cube, with many interrelated parts that must fit together.