Hassan Aboud, speaking to Al Jazeera from an undisclosed location inside Syria, said the group would "absolutely not" take part in the talks in Switzerland, planned for January 22.
World powers hope that the Syrian opposition and the government of President Bashar al-Assad will agree on a political solution to end the war.
Asked if his group would abide by an agreement, Hassan Aboud said: "Whatever comes out of it, is binding only on the Syrian National Coalition", referring to the main political opposition bloc.
"As far as we are concerned, we will continue the revolution until we restore our rights and our dignity."
Aboud said the groups that have agreed to attend the Geneva conference have "no leverage on the ground".
Founded two years ago, Ahrar al-Sham is a Salafist group which wants to implement Islamic law in Syria.
It is well-armed, with about 20,000 fighters. Some opposition groups say Ahrar al-Sham receives financial support from Persian Gulf states.