Abdullah al-Serhan died immediately after a vacuum bomb hit his vehicle outside of the city, the group said.
"Assad terrorists targeted today the car of The White Helmets on the road of Gharar in Daraa countryside. The terrorists used a thermal missile in their attack," Moatesem al-Hassan from Orient News reported.
"We can confirm you that head of the office of the rescue group Adbullah al-Serhan Abo Yassin was immediately killed."
The White Helmets have been frequently targeted in regime and Russian "double tap" bombings, when secondary air raids are launched on bombed-out targets as civil defence teams attempt to rescue the injured.
In late February, the White Helmets announced that one of its volunteers, Mohammed Dabdoub, was killed by regime artillery fire in Homs.
The White Helmets, of which there are approximately 3,000 across Syria, work to save and rescue civilians buried under the rubble.
The White Helmets a 40-minute Netflix documentary, won an Oscar for best documentary short this year.
No member of the group was present in Los Angeles in February to pick up the award, in part due to visa restrictions established by the Trump administration, but also due to Damascus' decision to cancel the passports of the filmmakers.
The Syrian war has killed more than 310,000 people, and displaced over half the population, including millions who have fled abroad since it began in March 2011 with anti-government protests.
— Putintin (@putintintin1) March 20, 2017
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