According to the operator, the airplane was cruising at flight level 250, about 1 hour and 40 minutes into the flight, when it encountered 30 to 45 seconds of severe turbulence. One of the three passengers was not seated and sustained serious injuries. The injured passenger reported that she struck her head on the overhead ceiling compartment, then fell onto the armrest. Another passenger assisted her to her seat, but the injured passenger had difficulty fastening her seatbelt. According to the operator, the crew reported the turbulence to air traffic control, advised the controller of the injury, and continued to the destination airport at the request of the passengers. At the destination airport, the passenger was met by emergency medical services personnel and declined additional medical treatment. The passenger reported that she sought medical treatment the following week and had a fractured rib. The pilot recorded the passenger’s seatbelt in the aircraft maintenance logbook. Mechanics performed a functional check of the seatbelt and found it to be operational, and the airplane was returned to service.