While confirming the accident, which occurred at 11pm, Kanyonyoo sub-location assistant chief Jacob Mueti said the matatu was ferrying people coming from a dowry negotiation ceremony in Mwingi.
According to Mr. Mueti, the 14-seater matatu lost control while attempting to overtake another vehicle.
Some of the passengers who sustained injuries in the accident are receiving treatment at Kitui level 4 hospital, where the bodies of the deceased were moved.
Other injured passengers were also taken to Matuu Level 4 Hospital in Machakos, and the Kenyatta National Hospital in Nairobi.