It is a deciduous large shrub or small tree growing to 5 m tall, with blackish bark and dense, stiff, spiny branches. The leaves are oval, 2–4.5 cm long and 1.2–2 cm broad, with a serrated margin. The flowers are 1.5 cm diameter, with five slightly creamy-white petals; they are produced shortly before the leaves in early spring, and are hermaphroditic and insect-pollinated. The fruit, called a "sloe" is a drupe 10–12 mm diameter, black with a pale purple-blue waxy bloom, ripening in autumn, and harvested in October or November - usually after the first frosts. They are thin-fleshed, with a very strongly astringent flavour when fresh.
It is frequently confused with the related cherry plum (Prunus cerasifera), particularly in early spring when the latter starts flowering somewhat earlier than P. spinosa. They can be distinguished by flower colour, creamy white in P. spinosa, pure white in P. cerasifera. They can also be distinguished in winter by the more shrubby habit with stiffer, wider-angled branches of P. spinosa, and in summer by the relatively narrower leaves of P. spinosa, more than twice as long as broad.
It is extensively planted for hedging and for cover for game birds. The small thorns of the plant are relatively common causes of minor wounds in livestock, and these wounds often fester until the thorn is expelled or removed.
A "sloe-thorn worm" used as fishing bait is mentioned in the 15th century work, The Treatyse of Fishing with an Angle, by Juliana Berners.
The expression "sloe-eyed" for a person with dark eyes comes from the fruit, and is first attested in A.J.Wilson's 1867 novel Vashti.