Seahorses are found all over the world, in tropical and temperate coastal waters. The main aim of this study was to characterize the Hippocampus reidi populations in different depth and hydrodynamism categories in Arraial do Cabo, Rio de Janeiro state. The study was developed through scuba diving, with transects of 20m x 5m in two different depth layers: shallow, between 0 and 5m, and deep, between 5 and 10m. Through visual surveys, individuals were identified according to species, sex, and size (height – HT). The density of H. reidi varied according to depth and hydrodynamism. The highest mean depth was found in the Cabo Frio Island – Ponta do Anequin, as the lowest depth was found in Praia do Forno – Ponta D’água. There is no significant difference in the size of individuals nor in the sex ratio between the depth categories. The sex ratio of H. reidi in the population of Arraial do Cabo was 1:1. The results obtained indicated that the population characteristics are similar between the two depth layers.