Brett McGurk, who served in the post for three years, resigned on December 22, following Trump’s decision to pull troops from Syria. The move also saw a resignation from defence chief James Mattis.
Ambassador James Jeffrey, the current special representative for Syria Engagement, will take the additional role of special envoy for the US-led coalition against Daesh effective Friday, the State Department said.
"With this additional responsibility, Ambassador Jeffrey will lead and coordinate US Department of State relations with the Global Coalition to defeat ISIS (Daesh) and Department efforts to implement President Trump’s announcement of a responsible US troop withdrawal from Syria," said Robert Palladino in a statement.
The US withdrawal comes on the eve of a possible Turkish military operation in northeastern Syria against the PKK/YPG terrorist group.
PKK is recognised as a terrorist organisation by the US, EU and Turkey, YPG is its Syrian wing.
McGurk worked under the past three US administrations, becoming one of a handful of those appointed by former US President Barack Obama to keep his post under President Donald Trump. He also served in the George W. Bush administration.