The United Nations’ chief mediator for Syria, Staffan de Mistura, said Wednesday that he would step down at the end of November, becoming the third diplomat to leave the job since Syria’s civil war began in 2011.

Mr. de Mistura announced his resignation as special envoy during a meeting of the Security Council after four years and four months in the role, at a time when the future of Syria is uncertain. His departure could complicate efforts to negotiate an end to the war as it is winding down.

While his predecessors left the job out of frustration, Mr. de Mistura, 71, said he was leaving for “purely personal reasons,” adding, “I will definitely not say goodbye or engage in reflections today.”

The conflict, which began as a popular uprising in 2011 before degenerating into civil war, has killed hundreds of thousands of people and displaced millions.