Ski boot comprising a rigid shell which is shaped so as to accommodate the foot of the user, and a rigid cuff which is shaped so as to surround the ankle of the user and is pivotally jointed on the shell so as to be able to rotate about a reference axis locally substantially coincident with the articulation axis of the ankle of the user; the shell being provided with a rear heel which is structured so to be able to connect/ couple in a rigid and stable, though easily releasable, manner to the heel-piece of a ski binding device; the boot being also provided with a cuff locking device which is structured so as to be able to alternatively block the cuff rigidly to the shell for preventing any pivoting movement of the cuff on the shell, and release the cuff from the shell so to allow the cuff to pivot freely about said reference axis; the cuff locking device being able to automatically lock the cuff to the shell to prevent any pivoting movement of the cuff on the shell, as a result of connection/coupling of the heel of the boot to the heel-piece (13, 43) of the ski binding device, and also to automatically unlock/release the cuff from the shell as a result of disconnection/release/separation of the heel of the boot from the heel-piece (13, 43) of the ski binding device; the cuff locking device comprising: a movable locking member which is fixed to the shell in the area above the heel of the boot, and is able to move towards and away from a coupling position in which the movable locking member is able to couple in a rigid and stable, though easily removable, manner to the cuff while remaining rigidly integral with the shell; and a command assembly which is structured so as to be able to automatically move and maintain the movable locking member in the coupling position while the heel of the boot is coupled to the heel-piece (13, 43); the ski boot being characterised in that the command assembly comprises a rigid vertical strut which extends substantially coplanar with the centerline plane of the boot, from the heel up to reach the movable locking member, and is fastened on the body of the shell with the capability of freely sliding /moving in vertical direction between a lowered position and a raised position; the lower end of the strut being positioned at the heel, and being structured so as to be moved/pushed upwards by the heel-piece (13, 43) coupling with the heel of the boot. a movable locking member which is fixed to the shell in the area above the heel of the boot, and is able to move towards and away from a coupling position in which the movable locking member is able to couple in a rigid and stable, though easily removable, manner to the cuff while remaining rigidly integral with the shell; and a command assembly which is structured so as to be able to automatically move and maintain the movable locking member in the coupling position while the heel of the boot is coupled to the heel-piece (13, 43); the ski boot being characterised in that the command assembly comprises a rigid vertical strut which extends substantially coplanar with the centerline plane of the boot, from the heel up to reach the movable locking member, and is fastened on the body of the shell with the capability of freely sliding /moving in vertical direction between a lowered position and a raised position; the lower end of the strut being positioned at the heel, and being structured so as to be moved/pushed upwards by the heel-piece (13, 43) coupling with the heel of the boot.