The Storegga tsunami, triggered ∼8150 yr ago by one of the largest submarine slides on Earth, flooded margins of the North Sea and the Norwegian Sea that were occupied by Mesolithic peoples. Sand deposits of Storegga age overlie the remains of coastal settlements in Norway and Scotland. The resulting casualties, however, depend on the season when the tsunami struck. Human exposure would have been least in summer and early autumn, when Mesolithic peoples went to the mountains to hunt reindeer. The hunters returned to the coast in late autumn to settle into their winter quarters. Here we use moss stems to show that the Storegga tsunami happened in late autumn. The Storegga tsunami deposits contain samples of the moss Hylocomium splendens. Some of these stems retain green color and chlorophyll that imply burial alive. H. splendens has a characteristic growth cycle where new segments branch off from previous year’s growth. The Storegga mosses have segments more mature than those of modern July and August samples, but as mature as those from October through December. The enormous Storegga tsunami thus occurred late enough in the year to have reached a large fraction of the people then inhabiting western Norway and northeast Scotland. For those who survived the tsunami, the loss and destruction of dwellings, boats, equipment, and supplies must have made the following winter very difficult.