The powerful Ahrar al-Sham rebel faction said the IS-free zone, planned along a stretch of border in the northern province of Aleppo, "is in the interest of the Syrian people".
The safe zone would have "positive effects, from the humanitarian, military, and political perspectives that will serve the interests of both countries", the group's online statement said.
Ahrar al-Sham called Turkey, a main backer of Syria's fractious opposition, "the most important ally of the revolution".
"The Turkish government and people have presented over the past four years support for Syria's revolution and its people," Ahrar al-Sham said.
"The strategic relationship between us and Turkey is necessary to face challenges and shape the future of Syria."
Ahrar al-Sham is part of a rebel coalition, including Al-Qaeda affiliate Al-Nusra Front, that has seized large swathes of territory from regime forces in Idlib province.
Its support for the IS-free zone puts it at odds with its ally Al-Nusra, which announced its opposition to the plan over the weekend.
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Al-Nusra said it would withdraw from frontline positions against IS to avoid helping the US-Turkish plan, refusing to coordinate with any groups implementing the zone.
Turkey entered the anti-IS alliance last month, allowing US warplanes targeting the jihadists to use an air base in southern Turkey, and agreeing to the IS-free zone in northern Syria.
Ankara hopes the zone will be safe enough to accommodate at least some of the 1.8 million Syrian refugees in Turkey.
But questions remain regarding which armed groups would be responsible for enforcing the zone on the ground.
According to Syria expert Thomas Pierret, "Ahrar al-Sham is for Turkey an efficient, loyal, and responsible partner".
"The relationship has always been good. (Ahrar) is more powerful today than other groups allied to Turkey," the University of Edinburgh professor said.
But, Pierret said, partnering with the Islamist faction would be "problematic" for the United States, which is providing support to groups it has vetted as "moderate".
Early in the campaign against IS, US-led air strikes hit Ahrar al-Sham positions, although the group has not been targeted since.