The Syrian Observatory for Human Rights said that the Ahrar al-Sham, Jund al-Aqsa and Nusra Front groups had taken the city on Saturday.
Idlib is only the second provincial centre to fall into ISIS hands, after Raqqa was seized by Islamic State (IS).
Syria's civil war, which began four years ago, has killed more than 200,000 Syrians and displaced 11 million.
The UK-based observatory said the terrorist groups seized the city after four days of intense fighting.
They also were able to besiege two Shia towns of Kafrayah and al-Fu'ah, which are located 7 kilometers of Idlib, and now there is a big concern about the fate of tens of thousands of Shiites living in the towns.
Syria has not confirmed the loss.
State television said: "The army is fighting fierce battles to restore the situation back to what it was."
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