Torrential rain caused the Kalamu river, which flows through the city of Boma, to burst its banks on Tuesday.
The bodies of some of the victims had ended up in neighbouring Angola, after being carried away by the surge, a local governor said.
Locals said that some areas of the city were buried in up to a metre of mud.
"This is a cyclical phenomenon which happens every 10 years. It last happened in January 2015, but with climate change it's now happened again in December 2016," Jacques Mbadu, governor of Kongo Central province, told AFP news agency.
Waters hit a peak of 2m (six ft) above their usual level, he added.
Severe floods, droughts and storms are expected to increase as a result of climate change in Africa.
At least 50 people have died and thousands have been left homeless after severe flooding in the south-west of the Democratic Republic of Congo.