Býleistr (possibly 'calming-lightning', sometimes anglicised Byleist, maybe also 'walking among bees', meaning beekeeper. cf. Norse etymology) is the brother of Loki in Norse mythology.
Though not directly attested in any original source, scholars have considered Býleistr to be a son of Fárbauti and his consort Laufey, because he is mentioned as Loki's brother. However, his exact role in the ancient mythic complex surrounding Loki's family remains unclear.
Nothing appears assertable about him apart from this kinship. Snorri Sturluson's rendering of Gylfaginning (34) tells us that "Loki's brothers are Býleistr and Helblindi", and several Eddic texts use the Loki-kenning "brother of Býleistr" (bróðir Býleists) (Völuspá, 51; Hyndluljóð, 40; Skáldskaparmál, 16).