Macrobrachium hainanense is a large palaemonid shrimp (total length up to 7 cm) present at high densities (>2 individuals m⁻²) in pools of low-order forested streams in Southern China. The present study investigated the reproductive biology and sexual dimorphism of this shrimp in Tai Po Kau Forest Stream (TPK) and Tai Shing Stream (TSS), Hong Kong. Recruitment occurred during the wet season (May to August). Macrobrachium hainanense, a truly freshwater species, has abbreviated larval development, and the study populations had smaller broods than euryhaline species of this genus. Most females probably produced only a single brood of 20–75 eggs each breeding season. They showed no size-specific fecundity. Egg development in the laboratory took over 53 days, and eggs grew larger during development. The reproductive output (RO) of females was 11% for each brood, lower than for most other species of Macrobrachium. Lifetime RO was 21%, assuming a single brood in each of two seasons. The population male-biased sex ratio was 1.7:1, although the size-specific sex ratio was 1:1. Females reached maturity at 15–17 mm carapace length (CL) while males matured later at 18–22 mm CL. Females bred at two and three years of age, while males bred three times at two, three, and four years. Interstream variation in size at onset of sexual maturity was found, with shrimps at TPK maturing later than at TSS. Discriminant Analysis revealed that mature males and females were distinct in their abdominal depths and carapace lengths with females having deeper abdomens (for egg brooding) and smaller carapaces. Juvenile males could be distinguished from females by the presence of the appendix masculina. The minimum lifespan for females of M. hainanense was approximately 29 months and for males, 48 months. Competition for females may be responsible for the longer lifespan and larger size at maturity of males.