This study provides novel information regarding the length–weight relationships (LWRs) and reproductive characteristics of the crowned seahorse (Hippocampus coronatus) living along the southern coast of Korea. Specimens of H. coronatus were collected from an eelgrass bed in Dongdae Bay, Korea, during September 2006–August 2007. In total, 133 specimens were collected (76 females, 57 males) and their lengths ranged from 2.41 to 9.31 cm and 2.41 to 9.32 cm for females and males, respectively. No significant difference was observed between the sex ratio of specimens collected in this study and the theoretical 1:1 ratio (χ²-test, P > 0.05). All LWRs were highly correlated (r² > 0.941, P < 0.001), and the overall slopes of the length–weight regressions were 3.067 for females, 2.808 for males and 2.955 for both sexes. Size at first sexual maturity was 6.93 and 7.33 cm in length for females and males, respectively. Monthly changes in the gonadosomatic index (GSI) indicated that the spawning period was from June to November. Total number of eggs ranged from 56 to 163 with mature eggs from 4 to 56, and the number of brood in the pouches of males ranged from 12 to 46.