The javelin grunter (Pomadasys kaakan) is commercially important fish species for both gillnet and bottom trawl fisheries in the southwestern waters of Taiwan. However, background information on the biology of this species is scarce. The objective of this study was to examine the reproductive parameters for the stock assessment and management strategy of P. kaakan. Samples were collected from three fish markets in southwestern Taiwan between February 2020 and January 2021. A total of 551 individuals were sampled, which included 217 females and 334 males. The sex ratio (female percentage) was 0.39, and males were more abundant than females. The fork length (FL) for the females ranged from 195.7 to 650.0 mm, and the body weight (BW) ranged from 128 to 3720 g. The length-weight relationship was as follows: BW = 9.0 × 10-5 FL2.69 (R² = 0.97，n = 217). The FL for the males ranged from 169.9 to 591.0 mm, and the BW ranged from 76 to 2265 g. The length-weight relationship was as follows: BW = 8.0 × 10-5 FL2.71 (R² = 0.99，n = 334). According to the fluctuation of the gonadosomatic index and the histological observations, the greatest number of mature females was collected between February and July, with a peak from March to April. The batch fecundity ranged from 5,480 to 277,368 oocytes (an average of 56,646 oocytes). The estimated 50% length-at-maturity (Lm50) was 395.8 mm for the females. In March, specimens with translucent eggs were observed, which implied that the southwestern waters of Taiwan are the spawning grounds of P. kaakan