The black tiger shrimp, Penaeus monodon, is the most economically important penaeid shrimp in Bangladesh for both culture and capture fishery. The culture of these penaeid shrimps is always risky due to different diseases and environmental stressors. This study aimed to assess the relative expression of the Penaeus monodon Rab7 gene in the post larvae (PL) of the P. monodon cultured under different salinity conditions. We challenged the PL of the P. monodon in aquariums with two different salinity and dissolved oxygen conditions during a 7-day test. The relative expression of Rab7 was upregulated during the experimental period of 36 hours. Average differences in the relative expression of Rab7 were 0.3-folds and 0.5-folds, and 0.4-folds and 0.6-folds after 18 and 36 hours of rearing in 2 and 54 ppt salinity treated shrimp PL, respectively, when β-actin was used as an internal control. On the other hand, when elongation factor 1 alpha (EF1α) was used for normalization, the average relative expression of Rab7's fold differences were 0.2-folds and 0.5-folds, and 0.3-folds and 0.6-folds after 18 and 36 hours of rearing in 2 and 54 ppt salinity treated shrimp PL, respectively. Therefore, our results suggested that the relative expression of the Rab7 was positively correlated with an increased level of salinity and challenge duration. The significant increase in the expression level of Rab7 in the PL of P. monodon exposed to high salinity stress (54 ppt) conditions indicated its functional role in osmotic stress in shrimps. However, further studies are necessary to elucidate the mechanisms of how Rab7 is upregulated in shrimp cells after applying different levels of salinity using a larger sample group.