A vehicle side airbag device including an airbag accommodated within a seatback of a vehicle seat, and inflates and expands between a seated vehicle occupant and a side door, the airbag includes: a main body portion configured to restrain at least from a shoulder portion to a chest region of the seated vehicle occupant; and an extension portion configured to extend toward a vehicle front side or a vehicle rear side at a portion of a height region of the main body portion, and wherein, as seen in a vehicle side view, at a vehicle occupant shoulder portion height region, the extension portion overlaps a vehicle side portion member that structures a portion of a vehicle cabin side wall and that projects-out further toward a vehicle inner side than a side door glass.