Officials say the Riverside Transit Agency bus was traveling westbound on Highway 74 near Perris at speeds of 45 mph when it began to hydroplane just before 9 a.m.
The bus reportedly veered into oncoming traffic, colliding with a Pontiac sedan.
Four vehicles were involved in the crash, including a tow truck.
Fourteen people were transported to local hospitals with injuries ranging from minor to moderate.
The driver of the car, a 48-year-old, was among those hospitalized.
The CHP said the bus was carrying 22 passengers, plus the driver, who was not hurt.
Officials are trying to determine whether or not the bus may have experienced brake failure due to the wet roads.