A vehicle seat with a side airbag device includes: a gas generating device attached to an inner side of an outer side frame in a vehicle-width direction, which is disposed in a side portion on an outside of a seat back in the vehicle-width direction; a side airbag which inflates and deploys to expand over the outer side frame from a vehicle front side of the outer side frame in the vehicle-width direction by receiving gas from the gas generating device accommodated in the side airbag; and an inflation width expansion portion which is provided in the side airbag and causes an inflation width of the side airbag in the vehicle-width direction to be greater outside a side surface provided on an outside of the outer side frame in the vehicle-width direction than inside the side surface.