The group would initially focus on content that was a danger to society, such as terrorism
Sixteen of the world's biggest advertisers have joined together to push platforms such as Facebook, Twitter and Google's YouTube to do more to tackle dangerous and fake content online.
The Global Alliance for Responsible Media will also include media buying agencies from the major ad groups - WPP, IPG, Publicis, Omnicom and Dentsu - as well as the platform owners, the group said on Tuesday at the ad industry's annual gathering in Cannes, France.
Luis Di Como, executive vice president of global media at Unilever, said it was the first time that all sides of the industry had come together to tackle a problem that had far reaching consequences for society.
"When industry challenges spill into society, creating division and putting our children at risk, it's on all of us to act," he said. "Founding this alliance is a great step towards rebuilding trust in our industry and society."
He said the group would initially focus on content that was a danger to society, such as terrorism.
Platform owners had taken steps to address the problems, he said, but their focus had been more reactive - tackling content after it appeared - than proactive.
The alliance will work together to develop processes and protocols to protect people and brands, he said.
Other brand owners in the alliance include Adidas , Danone, Diageo, Mondelez International, Nestle and Procter & Gamble .