Khaled Omar, 31, was with other members of his team helping people trapped in rubble in the embattled city of Aleppo, spokesman Abed Abdulrahman said by phone.
Two other White Helmets were also wounded.
"Mourning the loss of Khaled Omar. He saved countless lives, including the 'miracle baby,' " the group, also known as the Syrian Civil Defense, tweeted.
While Syria was being torn apart by bombings and violence, Omar stood close by to pick up the pieces. While others fled, Omar and his White Helmet colleagues ran towards burning, crumbling buildings to rescue as many people as possible.
The White Helmet, comprised of volunteers, have about 2,600 members including more than 50 women.
Omar's 2014 rescue of a 10-day-old baby, using his bare hands to clear out debris from an Aleppo home, became famous after it posted on YouTube.
In June 2014, Omar and his fellow White Helmets had rushed to a bombed out apartment building. One of the women there was trying to find her newborn in the rubble.
After hours of searching, an exhausted Omar and the others were ready to give up when they heard a cry beneath the debris, he told Vocativ in 2014.
"I thought I was being delusional because I was so tired," he told the website.
That afternoon, they followed the newborn's cries to where a bedroom on stood. He and his colleagues figured out that the baby was trapped underneath two slabs of what used to be the roof as well as pieces of the concrete wall.
For hours into the night, they gingerly removed rubble and held their breath every time they heard a groan or a creak of the slabs.
With a flashlight shining on a small hole, Omar finally saw the newborn's dust-covered head protruding from the debris. He had been under about 10 feet of rubble, Omar would later tell the BBC. The child wailed as Omar gently pulled the swaddling cloth and freed him from the rubble. The people around him cheered raucously and Omar's eyes welled with tears as he cradled the baby -- a fragile symbol of life amid destruction.
"There are many stories about working from the morning to the evening trying to save children," he told Vocativ. "The only difference is this rescue was caught on film."
The baby, named Mahmud, survived the ordeal. Omar got to hold the child one more time and Mahmud promptly fell asleep in his arms.
"If I die saving lives I think God would definitely consider me a martyr," he told Vocativ that year.
Omar's journey with the White Helmets
Omar was married and had a 3-year-old daughter, the White Helmets said.
In 2014, the rescuer went to the United Nations to plead for the Syrians' cause. His journey to New York was the only time he had ever been on a plane.
Last year, CNN's Dr. Sanjay Gupta interviewed some of the White Helmet members who were training in Turkey. Every one of them had a story about saving lives. At the time, the group said it had rescued more than 18,000 people from death.
"They have all chosen to risk their lives to save others and that makes every single one of them a hero," White Helmets organizer James Le Mesurier told CNN.
The members, like Omar, are ordinary people who worked as a barber, a blacksmith or a police detective before the unrest upended their lives.
Omar was killed Thursday in Aleppo, a city that remains in dire shape.
A doctor in Aleppo told CNN on Thursday they still have basic resources at the hospital, but they lack food, water and electricity. The power is on for four hours a day.
Patients have told him they heard helicopters dropping bombs nearby. It is unclear who is conducting the airstrikes but the Syrian opposition lacks aircraft, so he thinks it is either the Syrian military or the Russians.
Some of the last doctors remaining in rebel-held eastern Aleppo have written an open letter to US President Barack Obama
pleading with him to help stop the atrocities in the city.
"We do not need tears or sympathy or even prayers," the letter said. "We desperately need a zone free from bombing over eastern Aleppo to stop the attacks, and international action to ensure Aleppo is never besieged again."
A senior official in the Obama administration told CNN the White House has received the letter and that it's working through the UN to find "a diplomatic approach."
But such words and diplomacy were too late for Omar.
He told the BBC during his visit to the UN:
"We hope that the American government and people will remember the Syrian people who've suffered bombing and destruction, and that they'll help us."