The new recruits joined the ranks in Aleppo province Sep 23 and 24, the London-based NGO said.
At least 41 of the new fighters are from other Islamist factions such as the Nusra Front, the Al Qaeda affiliate in Syria.
In addition, the IS was joined by 23 young Syrians who did not belong previously to any group, in addition to nine foreigners, five of whom are Arabs and four are Europeans.
In June, the jihadis proclaimed an Islamic caliphate in the territories they control in Syria and Iraq, and about 6,300 people joined the ranks of the radical group in the following month which marked the widest recruitment process since its foundation in April, 2013, according to SOHR.
The US began last week launching airstrikes against IS-held positions in Syria, in which warplanes from other nations are taking part.