The Least Storm-petrel (Halocyptena microsoma) is a small seabird of the storm-petrel family Hydrobatidae. It is 13-15 cm in length, with a wingspan of 32 cm. It is the smallest member of the order Procellariiformes. The species is the only member of the genus Halocyptena although it is sometimes included in the genus Oceanodroma.

It breeds on islands off the Baja Peninsula and Gulf of California of Mexico in rock crevices or small burrows in soft earth and lays a single white egg. Like most petrels, its walking ability is limited to a short shuffle to the burrow. It is a colonial nester.

It spends the rest of the year at sea, reaching as far south as the tropical Pacific South America. It frequently can be seen well offshore the coast of southern California in late summer and autumn.

It feeds on mainly planktonic crustaceans, with a preference of larvae of spiny lobster. It feeds similarly to other Storm-petrels, picking food of the surface of the water while in flight.

The Least Storm-petrel does suffer losses on some of the breeding islands, particularly from feral cats and rats.