Current environmental changes can drastically affect aquatic freshwater ecosystems. Salinization is one such change that affects freshwater species due to osmotic costs, which induce a wide variety of behavioural responses, including altered response to predation. This is particularly important in larval anurans, known to modulate their morphology, physiology and behaviour in response to salinity. Although these responses are known for long-term exposure, the effects of shorter exposure to salinity, relevant for coastal biodiversity as a result of sea water intrusions and precipitation variation, have rarely been assessed despite their importance in understanding the speed at which impacts on biodiversity occur. In this study, we experimentally acclimated larvae of the spined toad, Bufo spinosus, for 2 weeks to freshwater or to moderate levels of salinity (4 g/litre). The response to predation, activity and spatial movements of each individual were then assessed both at the salinity of acclimation and after a change in salinity. We demonstrated that medium-term exposure to salinity decreased activity, decreased growth and increased mortality. In contrast, sudden changes in salinity increased the escape distance of tadpoles in response to a predation stimulus but decreased activity. These results suggest that exposure to both stressors (predation cues and varying salinities) led to increased activity presumably linked to a stress response. However, individuals exposed to a change in salinity or to moderate salinity for medium durations expressed similar behavioural responses, indicating that the detrimental effects of high salinity can occur rapidly. Future studies should investigate the dynamics of osmolality in larval anurans exposed to brackish water to assess whether these behavioural shifts are linked with hydromineral dysregulation, and how long after exposure this occurs.