Sepiella japonica is an important commercial marine cuttlefish that is widely distributed around the coastal waters of the North-West Pacific and Northern Indian Ocean. The survival rate of the cuttlefish living in aquaculture farms may be subject to salinity fluctuations due to typhoons, rainstorms and tidal water flows. Changes in salinity may cause a series of physiological and behavioral stress responses in cephalopods, e.g. osmoregulation, immune capacity, hormone secretion and stress-induced inking behavior, and can even lead to mass deaths due to osmotic stress. Thus, during the process of seedling production of cephalopods, it is important to maintain the water salinity at the proper levels. To better understand the stress responses and the adaptability of cephalopods to cope with salinity stress, the changes in the behavioral activity, histopathology, osmoregulation and immune related enzymes of Sepiella japonica at different salinities of 15, 21, 27 and 33‰ exposed to 48 h were investigated in this study. The results showed that the distance moved, average velocity, the frequency of accelerations, active time and manic time of Sepiella japonica increased significantly under high salinity stress (27‰) (P<0.05). The cuttlefish tended to swimming around the tank wall when they were exposed to high salinity (27‰). Compared to the control groups (21‰), cuttlefish from the 27‰ and 33‰ groups showed more serious disorder in the tissue structures of muscle, gill and liver, while no changes was found in the 15‰ groups. With the increase in salinity, the activity of Na⁺/K⁺-ATPase in the gill decreased firstly and then increased (P<0.05). The cortisol content in brain tissue in the 33‰ groups was significantly higher than that from the other treatment groups (P<0.05). The fluctuation of salinity could lead to the significant increase of activities of superoxide dismutase (SOD) and catalase (CAT) (P<0.05), the content of malondialdehyde (MDA) (P<0.05), and a significant decrease of activity of lysozyme (LZM) (P<0.05) in gills and liver. Based on these results, it can be concluded that the Sepiella japonica has good tolerance to low-salt conditions, but high-salt stress could cause serious inking behavior and anxiety-like behavior. In addition, the range of salinities used in this study (15‰-33‰) could cause adverse effects on osmoregulation and immune related enzymes for Sepiella japonica. The results of the study would provide a reference for elucidating the regulatory mechanisms used by cephalopods to cope with salinity stress, and also offer valuable information to hatchery managers to help keep appropriate culture environment for Sepiella japonica.