A rain sensor is provided for fastening on an inner side of a windscreen of a motor vehicle. The rain sensor includes a holding frame for fastening to the inner side of the windscreen, a sensor element, a housing part for housing the sensor element such that the sensor element is arranged in the housing part, and a holding device for engaging the holding frame, the holding device being fastened to the housing part, wherein the holding device is brought into engagement with the holding frame in a pre-mounted state, and the holding device presses the housing part with the sensor element onto the windscreen in a final mounted state.