Diplomats from the two countries said in a statement the truce is intended to exchange prisoners and recover the dead, adding that specific details will be agreed on later.
Minutes after it entered force at noon (0800 GMT), Armenia and Azerbaijan accused each other of breaching the ceasefire with new attacks.
The claims could not be independently verified.
The announcement of the truce followed 10 hours of talks in Moscow sponsored by Russian Foreign Minister Sergey Lavrov, who read the statement.
It stipulated that the ceasefire should pave the way for talks on settling the conflict.
The latest outburst of fighting between Azerbaijani and Armenian forces began on September 27 and left hundreds of people dead in the biggest escalation of the decades-old conflict over Nagorno-Karabakh.
The region lies in Azerbaijan but has been under control of ethnic Armenian forces backed by Armenia since the end of a separatist war in 1994.
The talks between the foreign ministers of Armenia and Azerbaijan were held on invitation from Russian President Vladimir Putin, who brokered the ceasefire in a series of calls with President Ilham Aliyev of Azerbaijan and Armenia’s Prime Minister Nikol Pashinian.
Since the start of the latest fighting, Armenia said it was open to a ceasefire, while Azerbaijan previously had made a potential truce conditional on the Armenian forces’ withdrawal from Nagorno-Karabakh, arguing that the failure of international efforts to negotiate a political settlement left it no other choice but to resort to force.
Russia has co-sponsored peace talks on Nagorno-Karabakh together with the United States and France as co-chairs of the so-called Minsk Group, which is working under the auspices of the Organisation for Security and Cooperation in Europe.
They have not produced any deal, leaving Azerbaijan increasingly exasperated.
Speaking in an address to the nation on Friday hours before the ceasefire deal was reached, the Azerbaijani president insisted on his country’s right to reclaim its territory by force after nearly three decades of international talks that “haven’t yielded an inch of progress”.
“Mediators and leaders of some international organisations have stated that there is no military solution to the conflict,” Mr Aliyev said.
“I have disagreed with the thesis, and I have been right. The conflict is now being settled by military means and political means will come next.”
Fighting with heavy artillery, warplanes and drones has engulfed Nagorno-Karabakh, with both sides accusing each other of targeting residential areas and civilian infrastructure.
According to the Nagorno-Karabakh military, 404 of its servicemen have been killed since September 27.
Azerbaijan has not provided details on its military losses. Scores of civilians on both sides also have been killed.
Shortly after the truce took force, the Armenian military accused Azerbaijan of shelling the area near the town of Kapan in southeastern Armenia, killing one civilian.
Azerbaijan’s Defence Ministry rejected the claim as a “provocation”.
The Azerbaijani military, in turn, accused Armenia of striking the Terter and Agdam regions of Azerbaijan with missiles.
Armenia’s Defence Ministry denied that.
