The commander was killed when the ISIS blew up his car in the al-Ghouta countryside of Damascus, Xinhua reported citing the pro-government al-Khabar TV.
The victim, Harun, was the head of the Badr Brigade, a faction of the so-called Islam Army, which has been deadlocked in battles with the ISIS over control of rebel-held areas on the eastern edge of Damascus.
Jihadi groups, the major power fighting the government troops of Syrian President Bashar al-Assad, have engaged in infighting over control of territories and strategic facilities, mainly in northern and eastern Syria.
The rebel-on-rebel battles have claimed the lives of 7,000 rebels and civilians since the beginning of this year.
The ISIS, the strongest among the rebel groups, has already announced the establishment of an "Islamic State" in areas under its control in Syria and Iraq, as the organization has seized large chunks of territories from both countries in recent months.