Raqqa Is Being Slaughtered Silently (RBSS) said Ahmad Mohammed al-Mousa was killed by a group of masked men in the rebel-held city of Idlib on Wednesday.
The group have no further details, but at least two other members have been killed by IS militants since 2014.
RBSS was honoured last month by the Committee to Protect Journalists (CPJ).
The New York-based organisation condemned Mr Mousa's murder.
"Just a few weeks ago, nearly 900 journalists, press freedom advocates, and supporters attending CPJ's International Press Freedom Awards stood together in solidarity with Raqqa Is Being Slaughtered Silently. Today we are all standing together again, this time in mourning," said CPJ Executive Director Joel Simon.
Earlier this year, IS claimed responsibility for the kidnapping of Mr Mousa's father and four other Syrians accused of being members of RBSS.
In October, RBSS member Ibrahim Abdul Qader was killed by IS militants, along with fellow journalist Fares Hamadi, in south-eastern Turkey.
IS subsequently published a video showing one of the men with his throat cut, and warned: "You will not be safe from the knife of the Islamic State. Our hand will reach you wherever you are."
RBSS is one of the few independent sources of news left in the northern Syrian city of Raqqa, which IS has controlled since August 2013 and serves as the de facto capital of the "caliphate" whose creation the group proclaimed in June 2014.
Another member of a Syrian activist group that reports on the activities of the jihadist group Islamic State (IS) in the city of Raqqa has been murdered.