The invention relates to a cosmetic refill (20) intended to be inserted in a removable manner into a housing (22) in a device (2) for treating the hair and to the associated treatment device (2). The cosmetic refill (20) comprises a body (23) that has a first wall (40) and a second wall (38) opposite the first wall (40), and at least one coupling relief (50, 52) that extends from each of the first wall (40) and the second wall (38). The coupling relief (50) on the first wall (40) is intended to cooperate with a first surface (46) of the housing (22) when the first wall (40) is introduced into the housing (22). The coupling relief (52) on the second wall (38) is intended to cooperate with a second surface (44) of the housing (22) when the refill (20) is brought into a use position by pivoting in the housing (22) about a pivot axis (B).