An occupant protection system for protecting a vehicle occupant sitting in a seat includes a seatbelt that includes a lap belt for arresting the pelvis of the occupant, and an airbag that is stored in the lap belt. The airbag as deployed includes a main inflatable portion for protecting an upper body of the occupant, and a head protecting portion that is located in a vicinity of an upper end of the main inflatable portion for protecting a head of the occupant. The head protecting portion includes a front inflatable portion for covering a front side of the head, and a pair of side inflatable portions that extend rearward from left and right ends of the front inflatable portion for covering sides of the head. Each of the side inflatable portions is configured to be deployed in such a manner that a space is formed between itself and the head.