Pashinyan had come under pressure with thousands of demonstrators protesting since Tuesday and demanding he resign over the ceasefire.
What does Israel have to do with the Azerbaijan-Armenian ceasefire?
Armenia's PM has called it a disaster but said he had no choice but to sign it to stave off a defeat.
Some Azeris regretted fighting had ended before Azerbaijan controlled all of Nagorno-Karabakh and were wary of the arrival of peacekeepers from Russia, which long dominated the region in Soviet times
The agreement, signed by Russia, Armenia and Azerbaijan, follows six weeks of heavy fighting and advances by Azeri forces.
The declaration has followed six weeks of heavy fighting and advancement by the Azerbaijan's forces.
Turkey, Russia discuss conflict as civilians run from Nagorno-Karabakh capital
At least 1,000 people, and possibly many more, have been killed since fighting broke out on September 27 in Nagorno-Karabakh.