A ski boot with a leg portion hinged to the foot portion so that the leg portion is able to rotate around a rotation axis (X-X) in relation to the foot portion. The ski boot includes a constraint mechanism for selectively blocking and/or unblocking the rotation of the leg portion in relation to the foot portion depending on whether or not the user wishes to pass from a skiing configuration to a walking configuration. The constraint mechanism comprises a rod having a fixed end hinged to the foot portion and a movable end which interfaces with the leg portion, a support attached to the leg portion and a guide connected to the support for guiding the movable end of the rod. The guide is movable in relation to the support and moves with rotation of the rod during movement of the leg portion in relation to the foot portion.