It targeted the arms store belonging to Hayat Tahrir al-Sham, an alliance led by Syria's former Al-Qaeda affiliate, the Syrian Observatory for Human Rights said.
The blast killed seven HTS fighters and four other people, the war monitor said.
"Initial information points to a car bomb," Observatory chief Rami Abdel Rahman said.
HTS controls the city of Idlib, as well as the surrounding anti-regime bastion of the same name.
Last week, the jihadist-led alliance took administrative control of the whole of the region -- which is made up of a large part of the Idlib province, as well as adjacent chunks of the Aleppo and Hama provinces.
But the Islamic State group also have a presence in the area.
Abdel Rahman said ISIS was likely responsible for the bombing, after HTS executed members of the extremist group the previous day.
One of the last parts of Syria beyond government control, the Idlib region has been protected from a regime offensive since September by a buffer zone deal agreed by regime ally Russia and rebel backer Turkey.
Before the HTS takeover, the region had been rocked by infighting between jihadists and rival rebels.
The area sees frequent clashes between factions, including targeted assassinations and bomb attacks.
Syria's war has killed more than 360,000 people since it started in 2011 with the brutal repression of anti-government protests.
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