The retailers' promise followed last month's collapse of a nine-storey factory complex outside Dhaka that killed 1,127 people in one of the world's worst industrial disasters.
Labour activists have campaigned for years for a Western retailer-led drive to improve factories where workers toil long hours in sweatshop conditions for paltry pay.
The retailers' commitment represents a "huge victory", said Kalpona Akter, head of the Bangladesh Centre for Workers Solidarity.
The agreement, backed by a Europe-based labour coalition called IndustriALL, is "going to benefit each and every garment worker", he said.
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Italy's Benetton, along with Spanish company Mango and British retailer Primark, had placed orders with plants based in the Rana Plaza complex which after the cave-in was found to have seriously violated construction laws.
"We welcome this agreement," said Atiqul Islam, president of the Bangladesh Garment Manufacturers and Exporters Association which represents 4,500 apparel factories.
"If they come forward and help repair, retro-fit or strengthen our factories it's good for all of us. It's a reflection of their long-term commitment to Bangladesh," Islam told AFP.
The agreement, due to be formally signed in coming days, commits retailers to have independent building and fire safety inspections and pay for repairs.
The full list of signatories has yet to be revealed.
But US-based PVH, owner of the Tommy Hilfiger and Calvin Klein brands, and Germany's Tchibo were among the first to support it, according to anti-sweatshop network The Clean Clothes Campaign.