The Britain-based Syrian Observatory for Human Rights and an Idlib-based activist who goes by the name of Mohammed al-Sayid said members of the al-Qaida-linked Nusra Front and other rebel factions captured the Wadi Deif base this morning.
A Twitter account run by the Nusra Front in Idlib province said fighters are now removing mines from the area after the "Wadi Deif camp was liberated."
The capture of Wadi Deif is a blow to the Syrian government that has managed to hold the besieged post for more than two years and repelled repeated attacks by opposition fighters.
The capture came a day after rebels and Nusra Front fighters took over seven government checkpoints around Wadi Deif and the nearby base of Hamidiyeh.
The Wadi Deif and Hamidiyeh bases outside the town of Maaret al-Numan have long been prized targets for the rebels, who have launched multiple sieges since 2012.
The Observatory said that at least 15 pro-government troops and eight opposition fighters have been killed in the clashes yesterday.