The United Nations peacekeeping mission was not able to give a toll, but said the accident appeared to have taken place amid strong winds and poor weather.
The boat was ferrying goods and at least 38 people across Lake Kivu to the town of Bukavu when it tipped over and sank at around 4 a.m. (0200 GMT), according to the provincial chief of police, General Augustin Luzembo.
“There were seven survivors,” Luzembo told Reuters by telephone from Bukavu, without giving any details as to why the boat sank.
An official from the transport ministry in Kinshasa said it was not yet clear how many people had died in the accident.
U.N. spokesman Alexandre Essome said the mission was sending a team to the scene.
Luzembo said the boat was travelling to Bukavu from Minova, near the town of Goma in the north, when the accident happened near the island of Ntaligeza, around 70 km (43 miles) from Bukavu.
Boats are an important mode of transport in Congo, a vast country where the road network is virtually non-existent. Accidents are common due to poor maintenance and overloading, and death tolls are often high as many passengers are unable to swim.