Coastal freshening in the Southern Ocean is expected to increase under projected climate scenarios. As a major environmental stressor, prolonged reduced salinity could pose a significant challenge to Antarctica's endemic echinoderms. Acclimatising to low salinity may be crucial for their continued survival as climate change accelerates, yet little is currently known about their capacity to do so. The sea star Odontaster validus and sea urchin Sterechinus neumayeri, two of the most ecologically important and abundant echinoderms of the shallow Antarctic seas, were exposed to reduced salinities (29 ‰ and 24 ‰) for at least 71 days after a stepwise dilution from 34.5 ‰. Feeding, faecal production (S. neumayeri only) and activity coefficient were significantly impacted at 24 ‰ and did not recover to control levels in either species. Oxygen consumption remained similar to control levels (34.5 ‰) across both treatments and species until day 85, when a significant increase was observed in S. neumayeri at 24 ‰. Coelomic fluid osmolality was near isosmotic with external salinities in both species, while coelomocyte composition and concentration were unaffected by reduced salinities (S. neumayeri only). Both species demonstrated the capacity to tolerate lower salinities that may be expected with climate change, with successful acclimation demonstrated at 29 ‰. Although survival rates were high at 24 ‰, significant reductions in mass and the failure of metrics to return to control levels suggest that long-term survival at 24 ‰ is unlikely, potentially impacting Antarctic food-web dynamics and ecological interactions.