The goby Parapocryptes serperaster is a bottom dweller fish widely distributed in monsoonal regions in Asia, but its reproductive biology is poorly understood. This study assessed important variables associated with P. serperaster reproduction through monthly field collection and histological analysis in the Mekong Delta, Vietnam over a year. This goby species is a multiple spawner as the ovaries contained oocytes at different developmental stages during the spawning season, coinciding with the spawning period in the wet season. Peak spawning occurred in September when the gonadosomatic index was close to the highest value. This goby shows sexual dimorphism in body size, and the average size at first sex maturation was 15.8 cm total length in the female and 16.3 cm in the male. Fish reproduction was characterised by high variation of fecundity and heterogeneous size of eggs in the ovary, indicating a possible adaptation to the monsoonal region. This study provides new understanding on the spawning pattern and reproductive biology of P. serperaster living in the mud flat habitat in tropical monsoon climate.