Abstract: The internal osmotic pressure of Portunus trituberculatus is susceptible to changes of water salinity, which affects its growth, development, and reproduction. This study explored miR-139555 and its potential target gene PtNBC (sodium bicarbonate co-transporter gene) in Portunus trituberculatus adapted to salinity stress. The PtNBC gene was cloned using RACE technology. The full length of cDNA was 5308 bp with the ORF of 3570 bp, encoding for 1189 amino acids. PCR results indicated that both miR-139555 and PtNBC genes were expressed in all tissues of P. trituberculatus with the highest expression in the gill tissue (P<0.05). The results of dual luciferase assay showed that miR-139555 can significantly inhibit the expression of PtNBC gene (P<0.001). Low salinity stress significantly affected the expression of miR-139555 and PtNBC genes, and the expression trend was significantly negatively correlated. Under low-salinity stress, miR-139555 showed an overall upward-regulated expression trend, reaching the maximum at 48h, which was 4.9 times higher than that of the control group (P<0.05); while PtNBC gene showed a significant downward-regulated expression trend and the expression level was lower than the control group at all time points with the lowest value at 12h, which was 0.06 times lower than that of the control group (P<0.05). Our results proved that PtNBC gene is the target gene of miR-139555, and miR-139555 may play an important osmotic pressure regulation function by regulating PtNBC gene in gill epithelial cells.