The study investigates the sexual dimorphism in 205 samples (female:98; male:107) consisting of newly deceased fishes seen in the fishnets of the commercial fishers and long-snouted seahorses (Hippocampus guttulatus Cuvier, 1829) obtained in two research projects, in the coasts of Aegean Sea. All samples were divided by their standard-length groups (I: 60-89 mm; II: 90-120 mm; III: > 121 mm) of morphometric measurements. The sexes of the species differed in terms of tail length (TaL), dorsal fin base length (DL), trunk depth (TD9-10), and trunk length (TrL) (P < 0.05). Accordingly, it was determined that the positively allometric growth TD9-10, DL, and TaL values in male seahorses were higher than those of the female seahorses. The females were longer than the males in terms of TrL. However, sexual differences according to the length groups in male seahorses were significant starting from the length group II. Sexual dimorphism seen in the body parts is more likely effective in the development of eggs that the males took from the females related with the sexual selection pressure, while the longer tails and dorsal fins suggested that male seahorses were more advantageous in balancing their body weight during mating behaviors (tail grabbing and courtship etc.). In this research, as H. guttulatus distributed in the coasts of the Aegean Sea showed differentiation starting from length group II, it is understood that, there was a sexual dimorphism in terms of TrL and TaL which determine the body size in mate selection.