The truck was ferrying merchandise, as well as people, back from a local market near the southeastern town of Gueckedou when the accident occurred Sunday, state-run RTG radio reported.
As it was crossing a bridge, the truck became unbalanced and tipped, sending people as well as bags of cement and sacks of rice crashing into the river. Many of the dead drowned, suffocated under the ballast-like merchandise, RTG said.
At least 20 people were injured and were taken to a hospital, the radio station reported.
Guinea, a country of 10 million on Africa's West Coast is deeply impoverished despite having half the world's supply of bauxite — the raw material used to make aluminum — as well as iron ore, gold and diamonds. Unable to afford the price of a bus ticket, many Guineans prefer to hitch rides on crowded trucks.