Razor clams Solen gordonis are commercially harvested during early January to late April in Sasebo Bay, Kyushu, Japan. The fishery has substantial impact on the population because of noncatch mortality being roughly equal to fishing mortality; nevertheless, a municipal project of Sasebo City aiming to improve harvesting and to commercially brand S. gordonis clams harvested from the bay has been in progress since 2012. Despite the increasing commercial demand and fishery impact on the population, information about its reproductive cycle remains unclear. To reveal the reproductive cycle of the population, 25–51 clams with no foot breakage (281 individuals in sum total) were collected from the bay at almost monthly intervals during February to December 2016, and their sexes and reproductive stages were histologically determined. The reproductive cycle of the population showed a period of reproductive resting during February to July, gametogenesis sharply advancing in September, and spawning occurring during October to December, with a peak in November. This cycle was well synchronized between sexes (sex ratio = 1:1). This study is also the first report of gonadal infection by trematode and Marteilia-like parasites in S. gordonis clams. In conclusion, the present study revealed the reproductive cycle of the S. gordonis population in Sasebo Bay and confirmed that spawning season closure for the population has already been achieved by the present harvesting schedule.