To allow more reliable detection of occupancy of a seat by a person, the invention includes a detection device for detecting occupancy of a seat, in particular of a vehicle seat or motor vehicle seat, comprising at least one elongate light guide having at least one elongate core element and at least one elongate sheath at a distance from the core element. The light guide is deformable by mechanical actuation means in at least one direction running perpendicular to the longitudinal axis of the light guide by the deformation brought about by the seat being occupied causes the light to exit from the light guide on the side opposite the mechanical means.