Ibrahim Abdul Qader, 20, and friend Fares Hamadi "were found beheaded at the friend's house this morning," Abu Mohammad, a founder of the activist's "Raqa Is Being Slaughtered Silently" group, told AFP via the Internet.
The group, which documents abuses in areas under IS control in Syria, accused the jihadist organisation of the murders on its Facebook page.
According to Abu Mohammad, both men were from Raqa city, the de facto capital of IS in Syria. Hamadi was also in his early twenties.
Members of the activist group had been killed inside Syria in the past, but this is the first time a member has been killed outside the country, Abu Mohammad added.
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Turkey's Dohan news agency reported today that "two Syrian journalists were beheaded" in Sanliurfa, and that seven Syrians had been arrested by Turkish police.
Sanliurfa is 55 kilometres from Turkey's border with Syria's Raqa province, a major IS stronghold in the country.
Turkey has long been accused by Syrian opposition activists, Kurdish fighters and sometimes even Western partners of allowing IS members to slip back and forth across its 911-kilometre frontier with Syria.
The Raqa is Being Slaughtered Silently (RBSS) group was formed in April 2014, not long after IS fighters seized control of Raqa from other opposition groups.
The city became the first provincial capital to fall from regime control in March 2013 when an array of rebel groups seized it.
But since IS forced those groups out, it has tightly controlled the city, with activists providing a rare source of information about life in the city and surrounding province.
Activists documenting IS abuses have regularly been targeted by the group, which deems them "infidels" punishable by death.
Shortly after the group was formed, IS carried out a campaign of arrests in Raqa, rounding up dozens of people.
At the time, the group's members insisted they would continue their work nonetheless.