This study was conducted in Armação de Búzios, Brazil, a municipality where ecosystem degradation has been observed following large increases in tourism and population growth. The goal of this study was to determine seasonal variations in three Búzios populations of the long snout seahorse Hippocampus reidi. Monthly dives were conducted from November 2011 to October 2013. All three subpopulations had low densities of seahorses and no seasonality. The sex ratio differed at each site. The most commonly used microhabitats were the sponge Aplysina fulva and the seaweed Sargassum sp. There was no significant difference in temperature and salinity. The environmental trends could not explain the variation in seahorse density at the three beaches. The population showed no seasonality and no further decline.