The narrow-barred Spanish mackerel (Scomberomorus commerson) is an economically important fish species in Taiwan. Nevertheless, its reproductive biology in the region is poorly understood. This study provides the first information on the reproductive biology of this species in the central Taiwan Strait. In total, 1031 specimens were collected from January 2016 through August 2017, and the sex ratio (0.55) was significantly different from 0.5 (p = 0.0016). On the basis of progressive changes in ovary size, ovary histology, oocyte diameter, and gonadosomatic index, the spawning season was estimated to extend from March through August, peaking between March and May. The fecundity (F) and batch F (BF) were estimated to range from 193,212 to 9,810,450 and 91,117 to 2,077,581 eggs per fish, respectively. The relationships of F and BF to fork length (FL) were estimated as follows: F = 0.0247FL^4.005 (r² = 0.57, n = 110, p = 1.05 × 10⁻¹⁰) and BF = 0.00071FL^3.999 (r² = 0.514, n = 41, p = 0.0000149). Based on a logistic model, sizes at 50% of maturity were 68.0 and 68.4 cm in FL for female and male specimens, respectively. Hydrated and postovulatory oocytes were collected from March through June, suggesting that the Taiwan Strait is a spawning ground for S. commerson. The proportion of reproductively active female fish with postovulatory oocytes during the spawning season was 0.15, indicating that spawning occurs every 6.5 days on average. The parameters measured in this study, which provide useful information about S. commerson, can facilitate management, conservation, and sustainable fishing of this species in the region.