The provincial governor, Antoine Masamba, said the vessel was carrying more than 400 people, with reports that about 200 passengers are still missing.
The majority of those on board were teachers going to collect their salaries, a local mayor told the BBC.
He said 11 children were among those who died when the old and overcrowded boat went down in strong waves.
The vessel, bound for Boliangwa, sank on Saturday evening almost 30km (18 miles) from the town of Inongo, its point of departure and capital of Mai-Ndombe Province.
The mayor of Inongo, Simon Mboo Wemba, said police had arrested the captain of the boat and the investigation was ongoing.
Officials warn the search and recovery operations on the lake could take between two and three weeks.
The BBC’s Gaius Kowene in the capital, Kinshasa, says boat accidents are frequent in DR Congo because of overloading and poor maintenance
DR Congo, which is about the size of mainland Western Europe, has a poor road network and many areas are inaccessible unless by air or boat – and the country’s only super-highway is the Congo River.
Last month, at least 167 people died in two boat accidents on Lake Kivu.
This prompted Félix Tshisekedi, who became president in January, to make it mandatory for passengers to wear life jackets.
But it is not clear if those on the boat that sank on Saturday were wearing any.