One witness said the truck appeared to have been speeding when the driver lost control and collided head-on with the minibus near Bissauzinho, around 15 kilometres (15 miles) from the capital Bissau.
Sources said the bus had been overloaded and that at least 10 of the injured were in a serious condition.
Both drivers were killed and the minibus was totally crushed, an AFP correspondent at the scene said.
Authorities issued an appeal for blood donors after the accident, which occurred around 8:00 pm (local time).