The redtail prawn, Penaeus penicillatus, is commercially important for bottom trawling and shrimp beam trawling in the southwestern waters of Taiwan. However, no histological study of this species based on wild samples has been conducted. In this work, the reproductive biology of P. penicillatus was studied based on monthly samples collected in the southwestern waters of Taiwan from December 2020 to December 2021. The total length (TL) ranged from 84.5 to 253.3 mm for 634 females, while body weight (BW) varied from 7.2 to 140.2 g. For 956 males, minimum and maximum sizes were 78.3 and 181.7 mm TL, respectively, while BW ranged from 6.0 to 53.9 g. The relationship between TL and BW was BW = 8.0 × 10-5 TL2.61 (R² = 0.93, n = 1,590), and there were no statistically significant differences between females and males. The overall sex ratio was 0.40 (X2= 65.21, p < 0.001), but females were significantly more common among individuals larger than 165 mm TL. In addition, monthly sex ratios fluctuated from 0.19 to 0.51 without a systematic pattern. Reproductive activity, assessed using histology and the gonadosomatic index (GSI), indicated that mature females could be found during February to May and August to November, with peak spawning occurring from February to May. For females, the minimum size at maturity and estimated length at 50% maturity were 87.3 mm TL and 169.3 mm TL, respectively.