Another 24 people were injured in the collapse at the construction site in southwest Bangkok early on Saturday morning, Transport Minister Suriya Jungrungreangkit said.
Suriya offered his condolences and said victims would be compensated by contractors and relevant agencies.
He said four workers and an engineer were killed, and no road users were affected as traffic lanes near the construction were closed at the time.
Bangkok police commander Siam Boonsom said authorities were still investigating the cause of the collapse and would pursue legal action against those found responsible.
Photos from the site showed large metal and concrete structures had fallen on the ground and completely blocked the road.
Construction of the 5km elevated road started in 2022, according to project information published on Expressway Authority of Thailand website.
Surachet Laophulsuk, governor of the agency, said they were working to clear the debris as quickly as possible but it could take a week.
He said contractors working on the project were suspended for 14 days pending the investigation.
Road and construction safety is a major problem in Thailand.
The site of Saturday's collapse is a part of a major road connecting Bangkok to Thailand's south, which has been a subject of controversy over its prolonged construction and frequent fatal accidents.
Suriya said the transportation ministry was in the process of issuing new regulations that would suspend contractors found liable for construction accidents and blacklist them from bidding on future government projects.
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