The attack occurred in a market in the town of al-Dana, located in the north of the province near the border with Turkey, according to the Observatory.
Three people under 18 were among the dead and the blast also injured at least 30 other people, it said. Another bombing in the town after midnight on Friday killed two people, it added.
Rebel groups in Idlib province have been sporadically fighting each other since early this year. Rebels have also accused the ISIS terrorist group of carrying out attacks in the area.
Idlib province is a major stronghold of rebels in Syria and is situated along the border with Turkey, one of the main backers of their rebellion against President Bashar al-Assad.
The United Nations and aid agencies have voiced concern about the humanitarian situation in Idlib, where large numbers of people live in poor conditions and face aerial bombardment.
Syria's civil war has lasted over six years, killed hundreds of thousands of people, driven millions more from their homes in a global refugee crisis and dragged in regional and world powers.