"I will myself be moving on as of the last week of November," Staffan de Mistura told the UN Security Council during a meeting on the crisis in Syria, AFP reported.
The Italian-Swedish diplomat, who became the UN's third Syria envoy in July 2014, said he was leaving for "purely personal reasons" and had discussed his plans to leave with UN Secretary-General Antonio Guterres.
"I am not laying down the charge until the last hour of the last day of my mandate," he said.
De Mistura will be traveling to Damascus next week to push for the creation of a committee to agree on a post-war constitution for Syria.
De Mistura was appointed UN envoy for Syria in July 2014 after veteran Algerian diplomat Lakhdar Brahimi resigned following the failure of peace talks in Geneva.
Brahimi spent two years in the position, stepping in after former UN chief Kofi Annan quit just six months into the role.
More than 360,000 people have died in the war in Syria, which began in March 2011.