The accident happened around 7:30am on Highway 82 just west of Starkville in Center Grove community.
Oktibbeha County fire coordinator Kirk Rosenhan says the 18-wheeler had stopped for a school bus.
The driver of the car, 30-year-old Chavonna Gibson, was taken to OCH Regional Medical Center. She suffered a broken hip and leg.
The school bus was not struck, but two other vehicles were damaged by debris from the crash.
Rosenhan says Gibson may have been partially blinded by the sun.