A vehicle side airbag device includes: an inflator for generating gas by activation; a bag body provided in a side portion of a seatback of a vehicle seat and configured to be inflated and deployed toward a vehicle front side relative to the side portion when the gas is supplied thereinside; and a partition portion partitioning the bag body into a front bag portion and a rear bag portion to be inflated at a higher pressure than the front bag portion and having a thicker inflation width in a vehicle width direction than that of the front bag portion, the partition portion being overlapping with a central part of a shoulder of a sitting occupant in an inflated and deployed state in a side view.