A new hermit crab–sponge association is reported from soft-bottom sediment at 35–50?m depth in St Francis Bay, off the south coast of South Africa, and the associated sponge, Suberites ambulodomus sp. nov. (Suberitida, Suberitidae), is described as a species that is new to science. This species grows a spirally curved chamber occupied by the hermit crab Pagurus liochele. Suberites ambulodomus sp. nov. differs from congeners in southern Africa by having tylostrongyle megascleres (in addition to tylostyles) and smooth centrotylote microstrongyle microscleres. In addition to these morphological characters, the molecular marker cox1 was used to confirm that all collected specimens were conspecific, and distinct from the recently described S. dandelenae Samaai et al., 2017 from South Africa’s west coast. This is the first report of a sponge–hermit crab association in southern Africa, which is suggested to be facultatively mutualistic