The storm petrel is a small bird, in length with a wingspan. It weighs , with an average of . It is square-tailed and has all-black plumage except for a snow-white rump that extends to the sides of the tail base and a broad white band on the under wings. Juveniles in fresh plumage can also show a narrow white bar on the upper wing. The plumage becomes dark brown rather than black as it becomes worn. No obvious differences between the sexes are seen, although in the Mediterranean subspecies, at least, most captured birds can be sexed using a formula which involves multiplying the wing length by the length of the white rump band; females are slightly larger and have a longer white rump than males. The Mediterranean subspecies has longer wings and a heavier bill on average than the nominate form, but neither sex nor subspecies can be determined by observation at sea.