More than 86,000 civilians were among the 301,781 people killed, the Syrian Observatory for Human Rights said.
The civilian toll includes 15,099 children and 10,018 women, the Britain-based monitoring group said.
Rebel fighters made up 52,359 of those killed.
A total of 59,006 Syrian soldiers have been killed, in addition to 48,048 other pro-government fighters from countries including Iraq, Iran and Lebanon as well as Syria.
Jihadists of the Islamic State group and the onetime Al-Qaeda affiliate now renamed the Fateh al-Sham Front accounted for 52,031 of the dead.
The Observatory said another 3,645 victims could not be identified.
The figure is an increase of nearly 9,000 on the last death toll published by the Observatory in early August.
The United Nations and the major powers have made repeated efforts to end the bloodshed in Syria but all have so far failed.
A new ceasefire brokered by Moscow and Washington went into effect at sundown on Monday and AFP correspondents and residents reported that it appeared to be holding on its first full day Tuesday.