An assembly (1) for measuring a relative humidity level inside a watch (2), comprising the watch (2) and a device (4) for determining the humidity level present in the enclosure (9) of a case (3) of this watch (2), the determination device (4) comprising a free space optical system and a control unit (7). The system includes: an emitter module (5) for emitting at least one light beam (8); and a receiver module (6) for receiving the at least one light beam (8); the control unit (7) being connected to the modules (5, 6) and the at least one light beam (8) emitted by the emitter module (5) defining an optical path partly comprised in the case (3) and extending as far as the receiver module (6) and the control unit (7) being configured to evaluate the absorption of the light beam (8) by the water vapour capable of being present inside the enclosure (9) of the case (3).