The US embassy said Sullivan stood down and left Moscow on Sunday after almost three years in office. Elizabeth Rood will head the mission until his successor arrives.
"We intend to announce our next Ambassador soon, but don’t have anything to preview at the moment," the State Department Press Office said in a statement.
Sullivan's departure was "planned and part of a normal diplomatic rotation," it said. He has served out his term after agreeing to stay in Russia following Donald Trump’s departure from office.
Sullivan has been managing "one of the most critical bilateral relationships in the world during unprecedented times," the Department said, adding that "in moments like this, it is critical that diplomatic contacts between Washington and Moscow continue to manage tensions."