The diplomatic relationship between the United States and Russia is more strained than it has been in the past.
Sullivan has worked for five U.S. presidents and will retire after four decades.
Donald Trump appointed Sullivan as the ambassador to Russia in December.
The funeral for Gorbachev, the last leader of the Soviet Union, took place on Saturday.
Sullivan's replacement. The Charge d'Affaires will be taken over by the deputy chief of mission until a replacement is found. Dozens of Biden-era diplomatic nominations are still stuck in the Senate.
Sullivan stepped down as tensions between the U.S. and Russia increased. The U.S. and other allies imposed sanctions on Russian officials and businesses in order to get Russia to pull back. The Justice Department said in June that the US alone has frozen more than $30 billion worth of Russian assets. Putin accused the US of prolonging the conflict. The US is trying to get the release of several Americans, including a former Marine and a basketball player.
The Department of Justice says the U.S. and Allies have frozen $30 billion of Russians' assets.