The maturation and spawning season of threeline grunt Parapristipoma trilineatum were sampled using 3,296 specimens collected around Mugi-oshima Island of Tokushima Prefecture, Japan. Gonadal maturation was observed to occur from early May in females, and the middle of April in males. The spawning season was estimated to occur from late May to early July in females, from late May to early June in two-year old males and from late April to the middle of June in males over two years old. The maturity and spawning season in this sea area is early in comparison with that in other areas of the past report. The main factor leading to the rapid development is considered to be the recent high water temperature in this sea area. Respectively, 0% and 100% of females are mature at one and three years old; 76.7% of females, histologically, and 58.3% in the development stage of the ovary are mature at two years old. Most males mature at two years old.