The Syrian Observatory for Human Rights told AFP that 174 civilians, including 41 children, had died since February 27 in areas where the ceasefire had come into force.
The truce does not include territory where jihadists like the Islamic State group or Al-Qaeda's local affiliate are present.
The Observatory said that in jihadist-held areas, 189 civilians had died over the past month.
The total of 363 dead represented a major drop from the previous month, when more than 1,100 civilians, among them 234 children, were killed.
And it was the lowest number of civilian deaths since November 2011 -- just eight months after the conflict erupted -- when 296 civilians lost their lives.
Signup to our newsletter and follow us on Facebook and Twitter!
"The difference is huge. This is a very positive development," Observatory head Rami Abdel Rahman told AFP.
Syria's conflict has left more than 270,000 people -- among them nearly 80,000 civilians -- dead since it erupted in March 2011 with anti-government protests.
The ceasefire deal, brokered by the United States and Russia, has brought relative calm to swathes of the war-ravaged country.
But fighting has broken out among rival groups and with jihadist factions.
According to Observatory head Rami Abdel Rahman, 122 rebel fighters have been killed since the deal came into effect.
A total of 637 government and allied fighters also lost their lives.
More than 739 Islamic State group militants and another 247 hardline Islamist or jihadist fighters died in the same period.