AbstractViviparous sea snakes (Elapidae: Hydrophiinae) are fully marine reptiles distributed in the tropical and subtropical waters of the Indian and Pacific Oceans. Their known maximum diving depth ranges between 50 and 100 m and this is thought to limit their ecological ranges to shallow habitats. We report two observations, from industry‐owned remotely operated vehicles, of hydrophiine sea snakes swimming and foraging at depths of approximately 250 m in the Browse Basin on Australia's North West Shelf, in 2014 and 2017. These observations show that sea snakes are capable of diving to the dim‐lit, cold‐water mesopelagic zone, also known as the ‘twilight’ zone. These record‐setting dives raise new questions about the thermal tolerances, diving behaviour and ecological requirements of sea snakes. In addition to significantly extending previous diving records for sea snakes, these observations highlight the importance of university‐industry collaboration in surveying understudied deep‐sea habitats.