The invention is related to an alpine ski boot having a rigid shell whose base has an external sole on which a shell base is mounted, which shell base is overlaid by an upper having a rear portion and a front portion. The shell includes an internal sole on its bottom and also receives an internal cushioning including a flexible and comfortable liner, interposed between the foot and the lower part of the leg of the skier and the rigid shell. The rigid internal reinforcement has an asymmetrical transverse section and is of an overall squared shape. In particular, it includes a substantially vertical area extending laterally on the side of the shell base turned inward corresponding to the internal malleolus of the foot of the skier, and extended by a horizontal area extending on its bottom so that the shell base is rendered non-deformable in these areas exclusively, in order to provide firm support during vertical pressures on the sole and/or lateral pressures on the internal side, resulting from the forces exerted by the skier on the boot and its upper during skiing, without at all changing the flexibility characteristics to the other areas.