This brings the number discovered since last August to 23.
The UN has been unable to examine the mass graves and cannot say if they were recently dug.
It estimates that over 400 people, including women and children, have been killed in clashes between the army and a rebel group.
Jose Maria Aranaz, the director of the UN's joint human rights office, told the BBC that it is important that "a transparent independent investigation take place" in order to reassure the population and to ensure the perpetrators are brought to justice.
The government is fighting to put down a rebellion by a group called Kamwina Nsapu, which began after a regional chief was killed.
Both sides have been accused of committing human rights violations.
Late last month two UN experts who were abducted after going to investigate reports of abuses in the region were found dead in shallow graves.
The UN has said it has found 13 mass graves in the Democratic Republic of Congo's central Kasai province since the beginning of March.