The Japanese murrelet or crested murrelet (Synthliboramphus wumizusume) is a small seabird in the auk family that occurs along the remote rocky coasts and in the offshore waters of Japan, and may also be found after the breeding season as far as Sakhalin to the north and in particular off South Korea. With a small and declining population, estimated as of 2017 to total 2,500–10,000 individuals, it is the rarest alcid, and the most at risk of extinction.
The Japanese murrelet is a smallish seabird 24–26 cm in length, with a wingspan of 43 cm, and weighing some 164–183 g. Its upperparts are blackish and bluish grey, its throat and underparts white, legs and feet a yellowish grey, and short, thick bill a bluish grey, the culmen being darker; the iris is a dark brown. The black head features a white stripe from the top of each eye that meets on the nape, although this is less visible in winter. It may be distinguished from the Ancient murrelet (Synthliboramphus antiquus), which also occurs in much of its range, in particular by its summer crest of black feathers 3–5 cm in length, although this again is absent in winter. Juveniles resemble winter adults, though the head and upperparts are somewhat browner.