The accident occurred at Pinying area, a few meters from the scene where 41 people were killed two months ago when a bus lost control and rolled on the Nairobi-Narok highway.
Traffic Commandant Samuel Kimaru says the van had lost control and was rolling downhill when it collided with the bus ferrying passengers from Kisii to Nairobi on a bridge at the foot of the hill.
All the nine people in the van were killed while those in the bus sustained minor injuries.
The accident comes a day after Transport Cabinet Secretary Michael Kamau announced tough penalties for PSV owners whose vehicles are involved in accidents.