The sin croaker (Johnius dussumieri) is one of the important commercial species for trawl fisheries in southwestern Taiwan. Monthly samples of specimens taken from the trawl landings at the Ziguan and Tungkang fishing ports from February 2014 to February 2015 added up to a total of 676 specimens (292 females and 384 males). The total length (TL) and body weight (BW) of the collected fish ranged from 137.2 to 290.4 mm and from 37.5 to 320.5 g, respectively. For females, the relationship between TL and BW was estimated as BW = 7.0×10-6TL3.120 (R² = 0.947, n = 292). Based on the macroscopic appearance, histological examination of ovaries, and gonadosomatic index for these specimens, the spawning season for the sin croaker in the coastal waters off southwestern Taiwan likely lasts from February to August and peaks from May to August. The mean fecundity and mean batch fecundity of this species were estimated as 111804±61471 and 30259±22015, respectively. The sex ratio (female/total) was 0.43. The size (in TL) at 50% maturity (L50) was estimated to be 221.8 mm. Logistic curves describing the relationship between the proportion of maturity at each length interval and TL were estimated as Pr = 1/(1+e5.42-0.0244TL).