In this paper, we describe two new and morphologically very distinct species of spider beetle in the genus Mezium Curtis from Namibia. One of these species, Mezium inflatum Philips, is characterised by a dorsally expanded pronotum formed by a setose covering with an irregular internal "skeletal" scaffold consisting mostly of an air-filled space and the presence of extremely long erect elytral setae. The other, Mezium nigriaenum Philips, has very dark coloured elytra with the remaining body parts bronze. Further, the pronotal surface is densely covered with irregularly oval shaped, concave setae reminiscent of the surface of a honeycomb and with elongate appressed setae that converge medially forming a flattened loose tuft. Mezium pseudafricanum is proposed as a junior synonym of M. africanum, new records for M. andreaei and M. glabrum in Namibia are listed, a reassessment of M. gracilicornis (Pic) is given, and a distribution map is provided.  Keywords: spider beetle, synonymy, diversity, Namibia, Angola, South Africa, caves, variation.