Although Arctic communities are very sensitive to global warming, direct evidence of the effects of high temperature on bottom communities is quite rare. We observed a mass mortality event (MME) of sponges by SCUBA diving in July and August 2018 along the coasts of Kandalaksha Bay, White Sea, and sub-Arctic. This event severely affected sponges from hard-substratum communities in particular, the demosponges Isodyctia palmata and Halichondria sitiens. Constant and exceptionally high temperatures throughout the water column (average temperature differences of 6.5 °C in July and 5.6 °C in August 2018, relative to the average temperatures in previous years at a depth of 20 m) may have led to an environmental context favorable to the MME. As was observed for the thermal anomaly, mortality was limited at the depth below a thermocline. However, it is not possible to ascertain whether temperature had a direct effect on organisms or whether it acted in synergy with a latent and/or waterborne agent. However, viewed in the context of global warming, there is an urgent need to rapidly set up monitoring programs of physical–chemical parameters and vulnerable populations in benthic communities through the Arctic Basin.