Rocas is the only atoll in the south-western Atlantic and comprises a shallow platform area of about 360 km2, distant 230 km from the continent and 130 km from Fernando de Noronha Archipelago, in Brazil. Since the 1970s, its marine flora has received the attention of several authors, giving rise to considerable and well-established knowledge about its algal biodiversity, with a total of 143 seaweed species reported for the area. Previous efforts stressed the interesting absence of the green algae Halimeda, usually abundant in tropical reefs and one of the most important seaweed groups in the structure of the tropical benthic community. Herein, we report for the first time the presence of Halimeda opuntia at Rocas, collected during an expedition in January 2012. Based on phylogenetic analysis, was proposed that H. opuntia was originated in the Indo-Pacific Ocean and that its establishment in Brazil was independent from populations of the Caribbean as a whole and the Bahamas in particular. Another hypothesis is the current scenario of increased frequency and intensity of extreme events along the Brazilian coast, which could explain the transport of propagules from adjacent areas towards the atoll.