The present study reports the Hippocampus patagonicus Piacentino & Luzzatto, 2004 occurrence in shallow rocky reefs. We found two individuals in two different Guaíba Island beaches, Sepetiba Bay, Rio de Janeiro, Brazil. We recorded two adults with similar dimensions as reported in other studies with the same species. Usually, they are not found in shallow waters, but the present study reports them associated with rocky reefs less than one meter deep. This is the first record for the species in that environment and in Sepetiba Bay.