Sea Mither, or Mither of the Sea, is a mythical being of Orcadian folklore that lives in the sea during summer, when she confines the demonic nuckelavee to the ocean depths. Each spring she battles with her arch-enemy Teran, another spirit of Orcadian legend capable of causing severe winter storms, to gain control of the seas and the weather. Eventually Sea Mither overcomes Teran and sends him to the depths of the ocean, but the effort of keeping him confined there along with her other benevolent labours during the summer exhaust her, until in the autumn Teran takes advantage of her weakness to wrest control from her once again.
Stories of the Sea Mither and Teran are among Orkney's oldest legends, perhaps invented to explain the vagaries of weather and other naturally occurring events. In Shetland, fishermen petition Sea Mither to afford them protection from the Devil.