The big-head pennah croaker (Pennahia macrocephalus) is an important commercial species for trawl fisheries in southwestern Taiwan. Nevertheless, the reproductive biology of the population in the region is poorly understood. In this study, a total of 1,322 specimens were collected from April 2015 through March 2016, and the sex ratio (0.57) was significantly different from 0.5 (p<0.05). For females, the fork length (FL) and body weight (BW) of the collected fish ranged from 114.4 to 213.3 mm and from 20.52 to 152.13 g, respectively. The relationship between FL and BW was estimated as BW = 7.0×10-6 FL3.1734 (R²= 0.95, n=768, p < 0.05). On the basis of progressive changes in ovary size, ovary histology, oocyte diameter, and gonadosomatic index, the spawning season was estimated to extend from April through September, peaking between June and July. The fecundity (F) and batch F (BF) were estimated to range from 19,298 to 17,688 and from 65 to 12,387 eggs per fish, respectively. Based on a logistic model, the FL size at 50% of maturity was 178.7 mm for the female specimens. Hydrated and postovulatory oocytes were collected from April through October, suggesting that the waters off southwestern Taiwan are a spawning ground for P. macrocephalus.