Branchotenthes robinoverstreeti n. gen. and n. sp. (Monogenea: Hexabothriidae) infects the gill filaments of the bowmouth guitarfish, Rhina ancylostoma Bloch and Schneider, 1801 (Rhynchobatidae) in the southwest Indian Ocean off Trafalgar, South Africa. The new species is most easily distinguished from other hexabothriids by the combination of having a symmetrical haptor, C-shaped haptoral sucker sclerites that are approximately equal in size, haptoral suckers that are equal in size and arranged in 3 tandem pairs that each straddle the longitudinal axis of the haptor, glandular-walled vasa efferentia that are dilated in the distal portion and unite medially before connecting to the vas deferens, a thick-walled male copulatory organ with an oblong proximal portion and an unarmed and finger-like distal portion, parallel vaginae that each possess a thin-walled and sperm-filled proximal portion that is strongly sinuous as well as a thick-walled and musculoglandular distal portion that extends sinuously anteriad, and an egg with a filament at each end. This is the first report of a hexabothriid from a sharkfin guitarfish (Rhynchobatidae). A key to the genera of Hexabothriidae is provided.