A vehicle side door structure is equipped with a door inner panel and a door outer panel. An impact beam is disposed between the door inner panel and the door outer panel. The impact beam is disposed in such a way that its lengthwise direction coincides with the vehicle front and rear direction. An impact beam extension is disposed on a front end portion of the impact beam. The impact beam extension is overlaid on a first opposing wall portion formed on a vehicle front and rear direction front end portion side of the door inner panel, is welded to the first opposing wall portion. Furthermore, the impact beam extension overlaps at least part of a through hole serving as an example of a weak portion formed in a second opposing wall portion of the door inner panel as seen from the outer side in the vehicle width direction.