An occupant protection system 10 includes: arm rests 22 that are disposed on both seat width direction sides of a seat back 18 and are rotatable between a storage position and a use position; slide members 24 that are slidable along the arm rests 22 and are equipped with belt guides 34; a retractor 28 that takes up webbing 38; and a control unit that, in a state before an occupant D is seated in a vehicle seat 14, causes the arm rests 22 to be held in the storage position and causes the slide members 24 to slide in the seat upward direction above the head of the occupant D, and that, when the occupant D is seated, causes the arm rests 22 to rotate to the use position, causes the slide members 24 to slide toward base sides of the arm rests 22, and causes the webbing 38 to be taken up by the retractor 28.