Despite monogamous mating systems and size-assortative mating observed in many seahorse (Hippocampus) species, sexual dimorphism is nonetheless observed in some. This study documents morphometry and allometry of the adult Lined Seahorse, Hippocampus erectus, and examines sexual dimorphism in these measures. Males exhibit longer standard lengths, shorter trunks with isometric growth, and longer tails with positively allometric growth. In contrast, females demonstrate positively allometric growth of trunk length and isometric growth in tail length. There is no observed dimorphism in the weight–length relationship, but it is positively allometric. Sexual dimorphism of body parts suggests sexually selective pressures, with longer trunks in larger females accommodating developing ovaries, and longer tails hypothetically garnering advantages for larger males in courtship behavior (tail grabbing), gestation (supporting brood pouches), and intraspecific competition (tail wrestling). Though monogamous mating systems predict sexual monomorphism, the dimorphisms observed are probably a result of selection pressures due to the conventional sex roles maintained in this species.