A collection of marine algae made by R.W. Schmieder, organizer of the Cordell Expedition to Isla San Félix in March 2002, and observations made by Sylvia A. Earle during Cruise 12 of the R/V Anton Bruun in December 1965 have demonstrated the presence of eight species not previously reported from this remote and desolate volcanic island. The number of named species recorded from Isla San Félix is thereby increased to 19 compared to 28 for neighboring Isla San Ambrosio. The eight newly recorded species represent Rhodophycota (Chondracanthus intermedius, Jania tenella, and Rhodymenia californica), Phaeophycota (Colpomenia sinuosa, Glossophora kunthii, and Hydroclathrus clathratus), and Chlorophycota (Chaetomorpha antennina and Codium cerebriforme). Considering the two islands as a group, ten species have been recorded only from Isla San Félix, 19 have been recorded only from Isla San Ambrosio, and nine have been recorded from both islands, giving a total of 38 species.The marine algal flora of these islands has a strong affinity with that of Islas Juan Fernández, sharing 30 species (79%), of which 15 are insular endemics. Only 11 species (29%) also occur on the Pacific coast of South America, of which seven are widespread while four are endemic to South America. Only one species, Padina tristromatica, is endemic to Islas San Félix y San Ambrosio.