The figure, covering the period from March 2011 to April 2014, is the first issued by the U.N.'s human rights office since July 2013, when it documented more than 100,000 killed.
The high death toll is a reflection of the brutality of Syria's conflict, which has transformed into a complex, multi-layered war where various factions fight against each other.
It also reflects the recent surge in deadly attacks by the al-Qaida-breakaway Islamic State group targeting rival militant groups, mainstream Western-backed Syrian rebels and Kurdish militiamen in northern Syria as it seeks to eliminate opponents and consolidate its hold on territory and resources.
Navi Pillay, the U.N.'s top human rights official who oversees the Geneva-based office, said the new figures are so much higher because they include additional killings from earlier periods, as well as deaths since the last report. The exact figure of confirmed deaths is 191,369, Pillay said.
"As the report explains, tragically it is probably an underestimate of the real total number of people killed during the first three years of this murderous conflict," she said.
Men comprised 85 percent of the victims, women more than 9 percent, while the sex was unknown in the remaining cases.
The records show at least 8,800 child victims, although the age of most victims is unknown.
The figures are based on information from the Syrian Center for Statistics and Research, the Syrian Network for Human Rights, the Violations Documentation Center, the Syrian government and the Syrian Observatory for Human Rights.
Pillay criticized the world's "paralysis" over the fighting in Syria, which "has dropped off the international radar" in the face of so many other armed conflicts around the world. Her spokesman, Rupert Colville, told reporters she was referring mainly to the standoff on the U.N. Security Council.
Russia has been one of Syrian President Bashar Assad's main allies and has used its veto power four times at the 15-nation Security Council to prevent international sanctions on Syria.
In January, Pillay's office said it had stopped updating the death toll, blaming the organization's lack of access on the ground in Syria and its inability to verify source material. Colville said the new figures were released now because the United Nations improved its confidence in the way the analysis is conducted.
On Thursday, the Syrian Observatory for Human Rights said the number of deaths has reached 180,000.