Unknown gunmen attacked the White Helmet members at a base in Sarmeen, in the rebel-held province of Idlib.
The motive for the killing remains unknown. The attackers stole two minibuses and walkie-talkies, the group said.
The US state department said it was "saddened and horrified" by the attack.
"These cowardly acts of masked men took the lives of civilian volunteers who work tirelessly as first responders in order to save lives in incredibly dangerous environments," a statement said.
The French foreign ministry also condemned the killings, while the UN Office for the Coordination of Humanitarian Affairs said it was "devastated" by what had happened.
There were emotional scenes as dozens of people joined the funeral for the victims.
Sarmeen is controlled by Hayat Tahrir al-Sham (HTS), formerly al-Qaeda's official branch in Syria. The alliance described it as an "ugly crime".
The UK-based Syrian Observatory for Human Rights monitoring group said all seven White Helmets had been killed by bullets to the head.
What are the White Helmets?
- Started early 2013 as a volunteer workforce
- It has about 3,000 members, including bakers, tailors, carpenters, electricians
- Some 130 have been killed
- Say they are neutral, have no political affiliation and save people from all sides of conflict
- Also do repair works, reconnect electrical cables and secure the buildings
- Run by donations, also helped by US Aid and Dutch foreign ministry
The White Helmets say they are non-partisan, but critics, often supporters of President Bashar al-Assad and his ally Russia, allege links to jihadist groups and have long claimed that the organization fabricates reports and rescues.
They were the subject of a recent Netflix documentary and nominated for the 2016 Nobel Peace Prize.
Photo by: EPA Caption: The motives of the attack are still unknown