The boat had been transporting logs on Lake Mai Ndombe in Bandundu province, the United Nations-sponsored Radio Okapi said.
Radio Okapi said the boat, owned by a logging company called Sodefor, sank at around 8:00pm (19:00 GMT) on Wednesday.
Despite its vast mineral wealth, roads are almost non-existent outside Congo's main towns.
Travel is often limited to aircraft and the boats that ply its huge network of rivers.
Accidents are frequent due to overloading, lack of maintenance, and lax enforcement of safety standards.
At least 14 people died in September after their boat sank on an isolated stretch of the Lualaba river in the southern province of Katanga.