The accident occurred at around 7am on Sunday at Kanga area, past Mtito Andei, when the bus collided head-on with the lorry.
Makueni County Commissioner Abdullahi Khalif had earlier confirmed 20 fatalities before the death toll rose to 22.
The bus was reportedly ferrying passengers returning from a burial ceremony when the accident occurred.
Several other passengers sustained injuries and were rushed to Mtito Andei Sub-County Hospital for treatment.
The bodies of those who died in the crash were taken to the hospital's mortuary.
The circumstances surrounding the accident are yet to be established.