"The Syrian Observatory for Human Rights has documented the execution by the Islamic State of 1,878 people in Syria between June 28 when it announced its 'caliphate' and December 27," the group said in a statement.
The Observatory, which relies on a network of activist and medical sources on the ground in Syria, is based in Britain.
It said the victims were shot dead, beheaded or stoned to death in the provinces of Aleppo, Deir Ezzor, Hama, Homs, Hasakeh and Raqa.
Of those killed, 1,175 were civilians who included four children and eight women.
The dead included 930 members of the Shaitat tribe which rose up against IS in the eastern province of Deir Ezzor in the summer.
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On December 17, the Observatory said a mass grave containing the bodies of 230 Shaitat had been found in the province.
The jihadists also "executed" 502 soldiers and pro-regime militiamen, the monitoring group said.
It said IS also killed around 120 of its own members, mostly for trying to flee to their home countries, and 80 members of the rival Al-Nusra Front, the Al-Qaeda affiliate in Syria.
Despite giving a breakdown, the Observatory believes the number killed by IS to be far higher, given that many have disappeared and remain unaccounted for.
The jihadist group often records such killings on video and posts footage on the Internet, which experts say is meant to sow fear among civilians and rival groups, as well as to attract new recruits.