Anas al-Dyab, a photographer and videographer in his early 20s, was a member of the White Helmets who also contributed to AFP.
He was killed in Russian air strikes in the town of Khan Sheikhun, rescuers and the Syrian Observatory for Human Rights said.
The White Helmets, rescue workers in rebel areas named after their distinctive hard hats, said the group "mourns the fall of a hero Anas al-Dyab, a volunteer and media activist with the Syrian Civil Defence Centre in Idlib," in a Twitter post.
An AFP journalist saw White Helmet members gather to bid farewell to their friend, whose body was laid on a thick red blanket.
The Damascus regime and its Russian ally have stepped up their deadly bombardment of the jihadist-run region of Idlib since late April, despite a September buffer zone deal to protect the region of some three million people from a massive military assault.
Khan Sheikhun, a town in the south of Idlib, has been particularly hit, forcing thousands to flee their homes there, according to the United Nations.
But Dyab "chose to remain with his fellow volunteers in Khan Sheikhun till today," the White Helmets said.
Raed al-Saleh, the head of the White Helmets, said Dyab was killed while "trying to show the world what's going on in Syria".
"It's a great loss," he said.
Dyab, who was single, leaves behind his parents and three brothers, one of whom is held by the Damascus regime, Saleh said.
Hiding in a cellar
The Observatory said Dyab was hiding in the cellar of a three-storey building with two members of the Jaish al-Ezza rebel group when the strike happened.
Also on Sunday, regime air strikes killed 10 other civilians including three children in other parts of the bastion, said the Britain-based monitor, which relies on sources inside Syria for its information.
Syria's former Al-Qaeda affiliate Hayat Tahrir al-Sham in January took full administrative control of the Idlib region, although other jihadists and rebels are also present.
The Idlib region is supposed to be protected by a September 2018 deal between Russia and rebel backer Turkey, but a buffer zone planned under that accord was never fully implemented.
The White Helmets, who are backed by the West, were nominated for the Nobel Peace Prize in 2016.
But Moscow and Damascus accuse the group of backing rebels and jihadists.
Syria's war has killed a total of more than 370,000 people and displaced millions since it started in 2011 with a brutal crackdown on anti-government protests.