Sea ice has a nonzero salinity that varies in space and time. This affects the sea-ice thermal properties as well as the ice–ocean salt and freshwater exchanges, which may influence the sea-ice mass balance and the polar oceans’ characteristics. However, current sea-ice models neglect or misrepresent the ice salinity. In this paper, we address the question of the importance of large-scale sea-ice salinity variations for the sea-ice mass balance and the upper ocean. To examine this question, we formulate salinity variations in the framework of the sea-ice thickness distribution theory, using a simple parameterization for brine entrapment and drainage. The latter is tested one-dimensionally and then included in a three-dimensional large-scale ice–ocean model, OPA9-LIM3, which is run over 1970–2006, forced by a combination of atmospheric reanalyses and climatologies. Owing to differences in the physical forcings, the model simulates Arctic and Antarctic sea-ice salinity fields that profoundly differ, with a seasonal cycle that is found in reasonable agreement with available ice-core data. Then, the role of salinity variations is analyzed by comparing the results of the simulation including the interactive salinity with several sensitivity runs using simpler representations of ice salinity. The simulated large-scale sea-ice mass balance and upper ocean characteristics are found to be quite sensitive to the representation of ice salinity. In the Arctic, salinity variations induce changes in ice thickness up to 1 m in some regions, due to modifications in the sea-ice thermal properties. Around Antarctica, including salinity variations increases the simulated winter sea-ice volume by up to 28% because of changes in ice–ocean interactions that stabilize the ocean. The model sensitivity to the sea-ice salinity is of the same order of magnitude as a 10% change in bare ice albedo. Given the importance of salinity on the simulated sea-ice characteristics, sea-ice salinity variations should be included in assessments of the response of the high-latitude oceans to ongoing and future climate change.