Flexible sheathing 100 for the protection of a piston- cylinder unit for a disc brake which is capable of being arranged in a retracted condition wherein the piston 10 is completely accommodated within the cylinder 9 and in an extended condition wherein the piston 10 is completely extracted from a caliper body 4 defining the cylinder 9, said piston 10 having a piston outer surface suitable to co-operate with a cylinder inner surface 9 of the caliper body 4 which accommodates the piston, said outer surface having a predefined piston diameter and said cylinder inner surface 9 having a predefined cylinder diameter, wherein said piston comprises a piston body and a head thereof 29 suitable to press against a pad 14 of the disc brake, this sheathing 100 comprising: - a sheathing body 100, which in the retracted condition, of a disc-shaped configuration which develops from an inner diameter to an outer diameter; - said outer diameter being greater than said cylinder diameter; said inner diameter being lower than the piston diameter and defining a sheathing portion 100 developing from said inner diameter to said piston diameter; - a first hooking ring being arranged along the sheathing outer diameter 100 suitable to be fixed to a caliper body 4; - a second hooking ring being arranged along the inner diameter of the sheathing 100 suitable to be fixed to a piston 10; - a plurality of folds consisting of folds that consist of portions of said sheathing body which are folded such as to have the surfaces thereof directly facing each other and such as to allow the extension of the sheathing (100) during the outward movement of the piston 10 from the cylinder 9 of the caliper body 4; characterized in that the plurality of folds is totally arranged on said sheathing portion which extends from the inner diameter to the piston diameter