"Eric Green ... steps down from his post and leaves the diplomatic service after more than three decades of service," the message reads.
Since the beginning of Joe Biden's presidency, he has served as senior director for Russia and Central Asia at the National Security Council.
National Security Adviser Jake Sullivan said Green's role in working to support Ukraine was "central."
Earlier, US Congressman from the Republican Party Lauren Bobert called on the US authorities to audit all funds allocated to Ukraine against the backdrop of the resignations of Ukrainian officials who are involved in corruption.