The Britain-based Syrian Observatory for Human Rights said the figures includes almost 117,388 civilians, among them more than 22,000 children.
The Observatory’s previous tally was issued in December and stood at more than 387,000.
Observatory head Rami Abdel Rahman said 2020 saw the lowest annual death toll since the war began with just over 10,000 deaths.
Battles slowed this year as a ceasefire held in northwestern Syria and attention turned to containing the coronavirus pandemic.
The Observatory also documented at least 16,000 deaths in government prisons and detention centres since the conflict erupted in 2011 after the brutal repression of anti-government protests.
It said, however, that the real number was likely higher because its tally does no include 88,000 people believed to have died of torture in regime prisons.
Today the Damascus government controls more than 60 percent of Syria after a string of Russia-backed victories against jihadists and rebels since 2015.
Among the regions still beyond its reach are the last rebel enclave of Idlib in the northwest, Turkish-held areas along the northern border, and northeastern parts of the country held by US-backed Kurdish forces.
The war has forced more than half the country’s pre-war population to flee their homes.
Some 200,000 people have gone missing, according to the war monitor.