Medical and police officials say more than 70 people were trapped in the vehicle, which veered off a road and fell into a lake late Monday. The truck was headed to a weekly market in Koudou, near Guinea's border with Sierra Leone.
The driver and injured passengers were taken to a hospital in Gueckedou in southern Guinea.
Humanitarian workers in Guinea blame the combination of overly packed trucks, poor roads and lack of traffic regulations for many accidents in the country.
The director of the International Federation of Red Cross in Nzerekore, Mamady Cisse, covers the region where the accident happened. He says accidents of what he calls "mixed transport" have become more common nationwide.
Cisse says the shortage of transportation causes people to risk rides on trucks meant only for commerce. When an accident happens, he says, it is not unusual for the people to be buried under the contents of the truck.
Cisse says what normally happens after these accidents is people say the accident is an act of God, the drivers are pardoned, and people move on thinking that the damage has already been done and cannot be prevented.
Because of the absence of traffic laws and regulations on trucks, the Red Cross official says no sanctions or fines are imposed by the authorities.
Cisse says the government needs to create legislation to regulate mixed transport and to fine drivers carrying too many people and products. He says there also needs to be a campaign to educate people about the dangers of overcrowding cars.
Local journalist Maseco Conde says in recent months accidents in the Guinean cities of Mamou and Kankan each claimed about 20 lives, but he says Monday's accident is the most deadly that have happened recently.
He says accidents commonly happen because of inadequate public transportation and bad roads. He adds the recent nationwide strike since the beginning of the year left the roads in even more disrepair than normal.
The strike was called off earlier this month when the government named a new prime minister, Lansana Kouyate. His appointment met union demands for an independent minister with expanded powers to rule over the poverty-stricken country.