A device (10) for fastening a window blind with a window blind end piece (12), which has a front surface (13), on which is arranged an axis segment (15). The axis segment (15) interacts with a mounting element (18), which can be rotated about an axis of rotation (44) of the axis segment (15), for opening and/or closing a holding mount (17). An axial guide (47) is provided, which fixes an axial position of the mounting element (18) in relation to the axis of rotation (44) of the mounting element (18) about the axis segment (15). In order to reduce the risk of premature wear and to simplify a change between left-sided or right-sided mounting, the device (10) has a positioning device (48), which is separate from the axial guide (47). The positioning device (48) is provided for the detachable fixing of the mounting element (18) in at least one rotation position of the mounting element (18) in relation to the axis segment (15).