A binding for a touring ski, a Telemark ski or a cross-country ski having “step in” automatic locking. The binding includes a front retaining element to cooperate with the front sole of a boot, a rear retaining element to cooperate with the rear part of the front sole and/or the rear sole of the boot, a tensioning element to tension the boot on the binding and to enable the heel to be freely lifted, a tensioning link placed under and connected to a rear retaining stirrup, and a fixed or retraction stop to cooperate with the tensioning link so as to enable the rear retaining stirrup to move back, tensioning the tensioning element, when the heel of the boot exerts a downwardly directed pressure on the rear retaining boot situated in an idle position.