UN Syria envoy Staffan de Mistura to quit post next month
He said he is quitting the job for personal reasons but will continue in his tasks as the UN secretary general's - Antonio Guterres - special envoy for Syria until the end of November.
"I am not laying down the charge until the last hour of the last day of my mandate," he said.
De Mistura said he would travel to Damascus next week to push for the creation of a committee to agree on a post-war constitution for Syria.
The UN-led effort to set up the constitutional committee - that will include regime officials, opposition members and members of Syria's civil society - has been resisted by Damascus.
In 2014, De Mistura became the third UN diplomat tasked with ending the Syria war which erupted in 2011, when Syrian soldiers defected from the military after a series of regime massacres.
Since then, around 500,000 people have lost their lives in the war - the vast majority civilians killed by regime and Russian bombing or shelling - while more than half of Syria's population have been made homeless.
De Mistura's predecessor, Lakhdar Brahimi, resigned after becoming frustrated by the failure of Geneva peace talks.
An alternative Turkish, Iranian, Russian peace initiative has weakened UN efforts to end the war.
Syria's regime has repeatedly defied UN efforts to establish a ceasefire or peace deal to halt the fighting.
A fragile ceasefire currently covers opposition Idlib province, where a regime offensive threatened the lives of thousands of civilians, following Russian and Turkish mediation.