Xantus's Murrelet (Synthliboramphus hypoleucus) is a small seabird found in the California Current system in the Pacific Ocean. This auk breeds on islands off California and Mexico. It is threatened by predators introduced to its breeding colonies and by oil spills.

The species is named for the Hungarian ornithologist John Xantus de Vesey who described it from specimens collected off Baja California.

This species is a small black and white auk with a small head and thin sharp bill. It resembles the closely related Craveri's Murrelet, with which it shares the distinction of being the most southerly living of all the auk species. It breeds on islands in the Channel Islands of California, the largest colony being on Santa Barbara Island, and also several islands off Baja California, including Isla Guadalupe. After the breeding season it disperses north at sea, usually to offshore waters, as far as British Columbia.

Xantus's Murrelet feeds far out at sea, often in association with large pelagic predatory fish like tuna, on larval fish like anchovies, sardines and Sebastes rockfish. Like all auks it is a wing-propelled diver, chasing down prey under the water with powerful wingbeats. There is some speculation that it may feed cooperatively in pairs, as it is almost always observed in pairs, even during the breeding season. It flies well, and can take off without taxiing.

It nests in small crevices, caves and under dense bushes on arid islands in loose scattered colonies. It returns to the colony only at night, laying two eggs which are incubated for about a month. Like other synthliboramphine murrelets (e.g. the Ancient Murrelet) the chicks are highly precocial, leaving the nest within two days of hatching and running actively towards the sea, where the parents call to them. Once at sea the family swims to offshore waters.

Little is known about the time at sea due to difficulties in studying them. A female shot at Isla Guadalupe at the end of June was moulting its primary remiges (flight feathers) and unable to fly.

By the end of the 20th century, Xantus's Murrelet was considered to be among the most endangered species of auk.. This has changed since, but not because the present species' status has improved, but because other auks (e.g. Kittlitz's Murrelet) have become rarer. The species is currently classified as Vulnerable (VU B2ab(iv,v); C2a(ii)) by the IUCN.

Xantus's Murrelet is mainly threatened by oil spills, as much of its population lives near the busy shipping lanes connecting Los Angeles to other ports. Because a large part of its small population nests in such a small area a single catastrophic oil spill could have far reaching implications. It is also threatened by introduced species such as rats and feral cats; this threat has been lessened lately by efforts to restore its habitat by removing introduced predators. In one case the population of rats was removed from Anacapa Island by the use of poisoned bait, the money for which being paid by a trust fund from an oil spill settlement.