James Le Mesurier, founder of the Mayday Rescue group, was found dead early today near his home in central Istanbul's Beyoglu district, a neighbour said.
A security source said it was believed that Mr Le Mesurier had fallen from the balcony of his home office and his death was being treated as a suspected suicide.
A third person, a diplomat, said the circumstances around his death were unclear.
The White Helmets, known officially as Syria Civil Defence, have been credited with saving thousands of people in rebel-held areas during years of bombing by Syrian government and Russian forces in the country's more than eight-year civil war.
White Helmets members say they are neutral. Syrian President Bashar al-Assad and his backers describe them as tools of western propaganda and Islamist-led insurgents.
Mayday Rescue, a not-for-profit organisation, began its operations in 2014 and established an office in Istanbul in 2015 to support its Syria project. Its projects have been funded by the United Nations and various governments, its website said.
Mayday Rescue did not immediately respond to an emailed query about Le Mesurier.
The security source said Mr Le Mesurier's wife told police that she and her husband had taken sleeping pills around 4am and went to bed.
She said she was later woken by knocking on the door and discovered that her husband was lying on the street surrounded by police, the source added.
Mr Le Mesurier told Al Jazeera in 2015 that he had begun training and supporting the organisation in early 2013 alongside Turkish rescue experts, starting with "a single team of 20 people".
"I was working in Istanbul... and got together with a group of Turkish earthquake rescue volunteers," he said.
The White Helmets quickly expanded, and are credited with saving tens of thousands of lives during Syria's conflict.
A documentary about the group won an Academy Award in 2017.
The White Helmets have become a favourite target of pro-Syrian and pro-Russian groups. They have accused the group of supporting terrorists in Syria and doctoring footage of atrocities committed by regime forces - claims strongly denied by its supporters.