In a vehicle side airbag device, when inflator operates, gas is supplied to left and right chambers of rear bag portion, and gas is supplied from communication opening through between a left and right side partitioning portions directly into a front bag portion. When inflation and expansion of the chambers are completed, the left and ride side partitioning portions are pushed against one another by internal pressures of the chambers, and the communication opening is closed-off. A valley, that is recessed toward a vehicle rear side, is formed at a front surface of the rear bag portion. At a time when the rear bag portion interferes with a vehicle occupant who is at an improper position, this valley is expanded due to the left and right chambers moving apart from one another in a vehicle left-right direction.