Twenty macroalgal species are added to the flora of coral reefs of the Rosario Islands, Colombian Caribbean Sea. Four species of Chlorophyta, three species of Phaeophyta and thirteen of Rhodophyta. Among the mentioned species, the green algae Udotea caribaea Littler et Littler and Batophora oerstedii J. Agardh and the red algae Rhodogorgon ramosissima J. Norris et Bucher, Rhodogorgon carriebowensis J. Norris et Bucher, Hydrolithon borgesenii (Foslie) Foslie and Champia vieillardii Kützing, are reported for the first time for the Colombian Caribbean; and Lithophyllum brachiatum (Heydrich) Lemoine and Antithamnion percurrens Dawson for the Atlantic Ocean. Taxonomic changes are made in some species previously reported for the islands, and the probable place of origin and temporal presence of Sargassum fluitans Borgesen and Turbinaria turbinata (Linné) Kuntze are explained.