: The ichthyofauna of Lake Tanganyika consists of 12 families of fish of which five belong to Siluriformes (catfishes). Studies on siluriforms and their parasites in this lake are very fragmentary. The present study was carried out to help fill the knowledge gap on the monogeneans parasitizing the siluriform fishes of Lake Tanganyika in general and more particularly Clarias gariepinus. The gills of a specimen of Clarias gariepinus (Clariidae) were examined for ectoparasites. We identified the monogenean Gyrodactylus transvaalensis (Gyrodactylidae). This is the first time this parasite was found infecting gills. We are the first to observe a large spine in the male copulatory organ of this species, and to provide measurements of its genital spines; this completes the description of the male copulatory organ which is important in standard monogenean identification. This is the first monogenean species reported on C. gariepinus at Lake Tanganyika and the third known species on a representative of Siluriformes of this lake. It brings the total number of species of Gyrodactylus recorded in Lake Tanganyika to four. Knowing that other locations where this species has been reported are geographically remote from Lake Tanganyika, we propose a “failure to diverge” phenomenon for G. transvaalensis.