The president announced this in a decree on Monday, which came after a reshuffle in the Ukrainian leadership.
At the same time, Stefanishyna says in a post on Facebook that the decision to resign was her own and "depends on personal circumstances."
She also says that the post in Washington has been the "greatest honor" of her life.
Olha Stefanishyna, who was previously Deputy Prime Minister and Minister of Justice, was appointed ambassador to the United States in August last year.
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