The primary objective of the present paper is to quantify the biogeographic affinity of the marine flora of Rodrigues within the Indian Ocean by means of the Simpson Similarity Coefficient. A second topic explores the observation that certain widespread macroalgae are absent from the island, using an extensive set of macroalgal distribution data for the Indian Ocean. Once these common Indian Ocean algae have been identified and selected for their absence in the coastal waters of Rodrigues, the two causal hypotheses, (1) low habitat diversity and (2) an incomplete process of colonization, are investigated by interpreting the ecological descriptions of a large number of Indian Ocean specimens from the GENT Herbarium. Comparisons with two neighbouring island groups, namely Mauritius and the Seychelles, show that the habitat preferences of these widespread macroalgae determine their island distributions. The biogeographic analysis shows that the species-poor marine flora of Rodrigues (179 spp.) is a subset of the tropical East African flora (Tanzania, Kenya, Mauritius and the Seychelles). The general trend of biogeographic affinities with regions within the Indian Ocean is best reflected by the Rhodophyceae, which account for (1) the highest species diversity and (2) the largest proportion of species with specific (restricted) distribution patterns (high biogeographic resolution). Regarding the explanatory hypothesis for the low species richness, the results of an analysis of the habitats of selected species favour the idea of limited habitat availability (with the consideration of potential undersampling of exposed shores with hard substrata).