Ali Saqr, 20, had reported his mother, Lina, to IS authorities in Raqa “because she tried to persuade him to leave IS and flee the city,” the monitor said.
Authorities subsequently arrested the woman and accused her of apostasy, the monitoring group said.
On Wednesday, she was shot to death by her son “in front of hundreds of people close to the mail service building in Raqa city,” the Observatory added.
Observatory head Rami Abdel Rahman said the woman, who was in her forties, was living in the nearby town of Tabaqa but worked in Raqa city.
The incident was widely condemned online by social media users.
Raqa is the de facto Syrian capital of IS’s so-called “caliphate,” the territories it controls in Syria and Iraq where it imposes its harsh interpretations of Islamic law.
The extremist group has been accused of carrying out mass killings, torture, rape and sexual slavery.
A US-led coalition has been striking the jihadists in Syria and Iraq for over a year.
More than 260,000 people have been killed in Syria’s war since it erupted in March 2011.