Five new species of the South African endemic spider beetle genus Pseudomezium Pic are described from the Western Cape province. They are Pseudomezium altimontanum, Pseudomezium glabrum, Pseudomezium periculum, Pseudomezium stenochasma and Pseudomezium swartbergense. Illustrations of the external morphology and male genitalia of each species are provided, as well as a distribution map. An updated key to all described Pseudomezium species is provided. Four previously established species names in the genus Pseudomezium are corrected because of their Code-incompliance. These names after mandatory nomenclatural correction are Pseudomezium brunneaurantiacum Smiley & Philips for "Pseudomezium brunneaurant"; Pseudomezium diteinum Smiley & Philips for "Pseudomezium diteinus"; Pseudomezium dolichotrichinum Smiley & Philips for "Pseudomezium dolichothrixus"; and Pseudomezium polyomorphum Smiley & Philips for "Pseudomezium polyomorphus." The diversity of spider beetles in South Africa is also summarised.