A folding clasp (10, 100, 200, 300) for a bracelet comprising: - a first blade (1, 101, 201, 301) and a second blade (2, 102, 202, 302), hinged to each other, the second blade comprising two longitudinal members (14, 14'), - a first fastening member connected to the first blade and intended to be connected with a first free end of the bracelet; - a second fastening member, intended to be connected with a second free end of the bracelet; - a locking mechanism for holding said first and second blades (1, 2) in the closed position of the clasp when said blades (1, 2) are brought together and folded over each other, said locking mechanism comprising: - an operating member (7, 7') provided to be activated by a user to unlock the locking mechanism and to open the clasp; - a device to adjust the length of the bracelet comprising: - a movable part to which one of said first or second fastening member is attached, said movable part being arranged to be longitudinally movable with respect to the longitudinal direction of the bracelet when adjusting the length, - a blocking device (33, 34, 37) arranged to block the movement of the movable part in at least one longitudinal movement direction, characterised in that said locking mechanism and said length-adjustment device are arranged such that an activation of said operating member by a user induces the unblock of said blocking device (33, 34, 37) and allows the displacement of the movable part in said at least one longitudinal displacement direction. - a first blade (1, 101, 201, 301) and a second blade (2, 102, 202, 302), hinged to each other, the second blade comprising two longitudinal members (14, 14'), - a first fastening member connected to the first blade and intended to be connected with a first free end of the bracelet; - a second fastening member, intended to be connected with a second free end of the bracelet; - a locking mechanism for holding said first and second blades (1, 2) in the closed position of the clasp when said blades (1, 2) are brought together and folded over each other, said locking mechanism comprising: - an operating member (7, 7') provided to be activated by a user to unlock the locking mechanism and to open the clasp; - a device to adjust the length of the bracelet comprising: - a movable part to which one of said first or second fastening member is attached, said movable part being arranged to be longitudinally movable with respect to the longitudinal direction of the bracelet when adjusting the length, - a blocking device (33, 34, 37) arranged to block the movement of the movable part in at least one longitudinal movement direction, characterised in that said locking mechanism and said length-adjustment device are arranged such that an activation of said operating member by a user induces the unblock of said blocking device (33, 34, 37) and allows the displacement of the movable part in said at least one longitudinal displacement direction. characterised in that said locking mechanism and said length-adjustment device are arranged such that an activation of said operating member by a user induces the unblock of said blocking device (33, 34, 37) and allows the displacement of the movable part in said at least one longitudinal displacement direction.