John Joseph Sullivan is an American attorney and government official who served as the United States Ambassador to Russia from 2020 to 2022, and who previously served as the 19th United States Deputy Secretary of State from 2017 to 2019. A member of the Republican Party, Sullivan served as Acting United States Secretary of State from April 1, 2018, to April 26, 2018, following President Donald Trump's dismissal of Secretary of State Rex Tillerson on March 13, 2018, until Tillerson's official successor, Mike Pompeo, was sworn in. Tillerson did not officially leave office until March 31, 2018. Sullivan, however, was delegated all responsibilities of the Secretary of State beginning March 13.

The deputy secretary of state of the United States is the principal deputy to the secretary of state. As of February 2024, the position is held by Kurt M. Campbell, who is serving under United States Secretary of State Antony Blinken. If the secretary of state resigns or dies, the deputy secretary of state becomes acting secretary of state until the president nominates and the Senate confirms a replacement. The position was created in 1972. Prior to July 13, 1972, the under secretary of state had been the second ranking officer of the Department of State.