Parapocryptes serperaster is a bottom-dwelling goby species widely distributed throughout monsoonal regions in Asia. A year-round study (2013–2014) on the length–weight relationship and body shape of this goby was conducted in the Mekong Delta, Vietnam, to understand its morphometric change and growth characteristics during the wet (January–May) and dry (June–December) seasons. The sex ratio in all collected specimens was not significantly different between wet and dry seasons. The female P. serperaster weighed (7.2–54.5 g) more than the males (6.7–49.0 g), but female length (12.5–21.2 cm) was not different from males (12.5–23.0 cm). Length–weight relationships were similar between female and male P. serperaster. Male and female fish showed a similar growth pattern between wet and dry seasons and the slope of the length–weight regression was close to the standard value (b = 3) for isometric growth. The body condition factor decreased as fish grew, but the seasonal impact on the condition factor depended on gender. The condition factor of male P. serperaster was greater than that of females in the wet season, but not significantly different between genders in the dry season. This goby shows isometric growth and provides useful information for our understanding on gobiid morphometric variation in areas with monsoonal seasonality.