The accident shortly before noon shut down northbound Interstate 5 in Commerce, east of downtown Los Angeles.
The crash involved a county bus carrying 25 inmates and three charter buses carrying Army personnel, California Highway Patrol Officer Ming Hsu said.
A car apparently cut off the jail bus and created a "domino effect" in which the other buses rear-ended each other, county fire Inspector Matt Levesque said.
One soldier was taken to a hospital in critical condition while two jail inmates and five military personnel received minor injuries, Levesque said.
The freeway was briefly closed while another sheriff's bus was brought up to remove the inmates.