This study constitutes a preliminary assessment of the abundance and species richness of sponges living on the brown seaweed Sargassum spp. from the southern Gulf of California (Mexico). The diversity and abundance of sponges associated with Sargassum was investigated from November 2006 to November 2007. In addition, it was examined whether temporal variations in the abundance of associations were influenced by changes in size and density of Sargassum as well as environmental factors such as water temperature and salinity. Twelve sponge species (4 Haplosclerida, 3 Poecilosclerida, 4 Dictyoceratida and 1 Clathrinida) were found growing epiphytically on the lower portions of Sargassum plants and the most frequent species were Callyspongia californica and Haliclona turquoisia (38% and 27%, respectively). The abundance of associations showed significant temporal and spatial variations, being higher from June to November (post Sargassum growth period), at depths from 0.5–2.5 m. The number of sponge species m−2 (both epiphytic and non-epiphytic) inside the seaweed bed was 4.8 species m−2, while outside it, the absence of hard substrate for attachment severely restricted the number of sponges. These results suggest that Sargassum provide substratum and habitat for the sponge community from this region, and that their seasonal growth patterns may influence the abundance and diversity of the associations.