Abstract: A liner for a sport boot shell and a method for making a custom fitted liner for a sport boot, are disclosed. The liner is designed to be worn inside a boot shell. In a preferred embodiment the liner includes inner and outer of flaps which wrap in opposite directions across a wearer's shin and forms a compound curve in a region in front of the wearer's ankle. The liner including flaps may be made from a single piece of thermoplastic foam material of uniform thickness. The design of the liner prevents the cuff of the liner from loosening around the wearer's shin and calf when the wearer's leg is flexed forward at the ankle. The shell can therefore be worn more loosely than is necessary with a boot liner of conventional design. The liner may be made by making an unfitted liner from a sheet of thermoformable material. The unfitted liner has a forwardly facing opening and wings extending forwardly on either side of opening.