A sports boot including an upper extending above the ankle joint and at least one rigid collar extending around the wearer's lower leg, the boot including a device for adjusting the volume of the collar to better accommodate the shape of the user's calf. The adjustment device includes a wedge, or collar extension, positioned inside the collar, the wedge being fixed to the collar by means of at least one pivot and by means of a fastening mechanism enabling the relative displacement of the wedge with respect to the collar as well as a deformation of the wedge. The fastening mechanism includes a plate equipped with a cylindrical body having an axis A1 and received in a first circular opening provided in the collar and a rivet received in a second circular opening provided in the wedge and received in a third circular opening having an axis A2 provided in the body, the axis A2 and the axis A1 being non-coaxial. The wedge is fixed by means of a first pivot and a second pivot, which are positioned on respective sides of the collar so that an axis Ap1 of the first pivot and an axis Ap2 of the second pivot are located on a first, substantially horizontal plane P1 and so that the fastening mechanism is not fixed on the first plane P1.