The growth and reproductive cycle of the jack mackerel (Trachurus japonicus) that were caught in the southwestern Sea of Japan during 2016-2017 were examined based on otolith annuli and gonad histology. Translucent and opaque zones on transverse sections on otoliths were identified and opaque rings were counted. The von Bertalanffy growth curve was expressed as follows: FL_t = 445{1 − exp (−0.149 (t + 1.74))} (0.250 < t < 7.67), where FL_t is the fork length (mm) at age t. The estimated body length at age t in the growth model was smaller than that of jack mackerel in the East China Sea. The spawning period was evaluated from May to September based on the monthly changes in the gonadosomatic index and histological observations. The spawning period starts later than in the East China Sea, but the minimum size at first maturity was similar to that of the jack mackerel in the East China Sea. The information generated in this research can contribute to the sustainable management of the species in the Sea of Japan.