The casualty was embedded with US forces when they were caught by an explosion on Thursday, the Ministry of Defence said.
An American soldier was also killed and five other soldiers were injured by a blast while deployed against IS - also called Daesh.
The explosion is reported to have taken place in the town of Manbij, 60 miles north east of Aleppo and close to the border with Turkey.
An MoD spokesman said the casualty was embedded with US forces fighting IS - also called Daesh - when they were caught by an explosion on Thursday.
He gave no information regarding their service branch, unit, gender or where in Syria the explosion happened.
But media reports suggested it was a male special forces soldier.
The spokesman said: “It is with regret that we must confirm that a member of the UK armed forces was killed by an improvised explosive device in Syria yesterday.
“The individual was embedded with US forces on a counter-Daesh operation when the incident occurred.
“The family has been notified and our thoughts are with them at this difficult time.”
US military spokesman Colonel Ryan Dillon told the Associated Press he could not immediately say who was behind the attack.
He added: “There is an investigation under way to identify who they could possibly be.
“We have our initial assessment and thoughts on that but we won’t provide until the investigation is complete.”