The group, which relies on activists, doctors and lawyers on the ground in Syria for its reporting, said July saw the Islamic State's largest recruitment yet.
"The number of IS fighters has passed 50,000 in Syria, including 20,000 non-Syrians," the group's director Rami Abdel Rahman said.
"July saw the largest recruitment since the group appeared in Syria in 2013, with more than 6,000 new fighters," he said.
There was no way to independently confirm the figures.
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Abdel Rahman said the new recruits in July included more than 1,000 foreign fighters from Chechnya, Europe and Arab countries, as well as Chinese Muslims.
He said most had entered Syria from Turkey.
Other recruits included defectors from the ranks of other armed opposition groups, including 200 from the Al-Qaeda affiliate in Syria, Al-Nusra Front.
The Islamic State grew out of Al-Qaeda's Iraqi branch, but has since parted ways with the group.
It initially cooperated with some of the armed opposition in Syria, but its abuses against rival rebels and civilians sparked a backlash that began this January.