Russian President Vladimir Putin said it would be 15km to 25km (9-15 miles) wide and come into force by 15 October.
Troops from Russia, an ally of Syria's government, and Turkey, which backs the rebels, will patrol the zone.
The UN had warned of a humanitarian catastrophe if the Syrian army launched an all-out assault to retake Idlib.
But after Monday's meeting between Mr Putin and Turkish President Recep Tayyip Erdogan in Sochi, Russian Defence Minister Sergei Shoigu said there would be no such operation in the region.
Idlib province and adjoining areas of Hama and Aleppo are home to an estimated 2.9 million people, including one million children.
A sharp increase in hostilities since the start of September and fears of further escalation have led to the displacement of more than 38,500 people.