The UK-based Syrian Observatory for Human Rights said that dozens more were injured in the city of Ariha.
People had been gathering at the mosque to break their Ramadan fast at the time, the group said.
The cause of the explosion is not yet known.
An unnamed senior non-Syrian member of al-Nusra was reportedly killed in the explosion at Salem mosque, according to the Observatory.
Ariha was one of the last government strongholds in Idlib when it was captured by rebels in May.
An Islamist coalition called the Jaish al-Fateh (Conquest Army), took control of the city after a rapid offensive, activists said.
Rebels had earlier claimed a number of cities in the province, including Idlib and Jisr al-Shughour.
The fall of the Ariha left most of Idlib province, bordering Turkey, in rebel hands.
Al-Nusra is among 13 Islamist groups that came together on Thursday to launch a massive assault on the northern city of Aleppo.
More than 230,000 people are believed to have been killed in Syria since the uprising against President Bashar al-Assad began in March 2011.
Some 11.5 million others - more than half of the country's population - have fled their homes.
At least 25 members of Syria's al-Qaeda affiliate, al-Nusra Front, including a leader, have been killed in a blast at a mosque in north-western Idlib province, activists say.