This study was performed to obtain information on the sex ratio, size at sexual maturity, and main spawning period of the butter clam Saxidomus purpuratus in Korea to contribute to the management of fisheries resources of this species. The age-specific sex ratios (female:male) of S. purpuratus were 1:2.20 (31.3% female) and 1:1.03 (49.2% female) in the 0+ and 1+ y classes, respectively, illustrating the initially higher percentage of males in the butter clam population. From the 2+ y class onward, however, the percentage of females in the population tended to increase with age. The shell length at sexual maturity was analyzed using a logistic regression model, and was determined to be 54.5 and 45.5mmfor females and males, respectively. The gonad index displayed a definitive pattern of seasonal changes, with high values in the spring and fall and low values in the summer and winter for both males and females. The butter clam S. purpuratus was found to have two spawning seasons per year, with the main spawning period being in May–June.