Adult northern elephant seals go to sea twice each year for periods of ≤8 months during which they range widely in the northern Pacific Ocean. Using new tracking technology, we showed that the species (and individuals) returned to the same foraging areas during postbreeding and postmolt movements, documenting the first double migration for any animal. We also showed segregation by sex during both migrations, the causes of which are unknown. Seals dove continually to depths of 250–550 m during both migrations and travelled linear distances of at least 18,000 (females)–21,000 km (males) during the 250 (males)–300 (females) days they were at sea. These are the longest annual migrations yet recorded for individual mammals. The double migrations apparently are modulated by the requirement for seals to return to land twice each year, to molt and to breed, although the reasons seals favor distant molting sites on the California Channel Islands over island and continental beaches nearer foraging areas are unknown.