Ambulance service chief Bogdan Oprita says the workers were digging in a 20-foot (6-meter) -deep ditch on Friday evening when the earth came down on them.
One man was pulled to safety and was hospitalized suffering from bruising and shock. Rescue workers dug through the night and the last body was removed early Saturday.
Dozens of people gathered to watch the 12-hour recovery efforts on Bucharest's main highway.
The underpass is designed to ease the flow of traffic from Bucharest's two airports to the city. Work began on it almost a year ago.
Labor safety officials are investigating.