The black redstart (Phoenicurus ochruros) is a small passerine bird in the genus Phoenicurus. Like its relatives, it was formerly classed as a member of the thrush family (Turdidae), but is now known to be an Old World flycatcher (Muscicapidae). Obsolete common names include Tithys redstart, blackstart (not to be confused with the species currently known as blackstart) and black redtail.

The first formal description of the black redstart was by the German naturalist Samuel Gottlieb Gmelin in 1774 under the binomial name Mottacilla ochruros. The species is now placed in the genus Phoenicurus that was introduced in 1817 by the English naturalist Thomas Forster. Both parts of the scientific name are from Ancient Greek and refer to the colour of the tail. The genus name Phoenicurus is from phoinix, "red", and -ouros -"tailed", and the specific ochruros is from okhros, "pale yellow" and -ouros.

The black redstart is a member of a temperate Eurasian clade, which also includes the Daurian redstart, Hodgson's redstart, the white-winged redstart and perhaps Przevalski's redstart. The ancestors of the present species diverged from about 3 million years ago (mya) (Late Pliocene) onwards and spread throughout much of Palearctic from 1.5 mya onward. It is not very closely related to the common redstart. As these are separated by different behaviour and ecological requirements and have not evolved fertilisation barriers, the two European species can produce apparently fertile and viable hybrids.

There are a number of subspecies, which differ mainly in the underpart colours of the adult males and, for some forms, calls; different authorities accept between five and seven subspecies. They can be separated into three major groups, according to morphology, biogeography and mtDNA cytochrome b sequence data.

P. o. phoenicuroides group. Basal central and eastern Asian forms which diverged from the ancestral stock as the species slowly spread west (c. 3–1.5 mya). Females and juveniles light grey brown.

P. o. ochruros group. Western Asian forms, whose lineage separated from the gibraltariensis group c. 1.5–0.5 mya. Females and juveniles intermediate.

P. o. gibraltariensis group. European population, which formed as a distinct subspecies probably during the last ice age. Females and juveniles dark grey.

The black redstart is 13–14.5 cm (5.1–5.7 in) in length and 12–20 g (0.42–0.71 oz) in weight, similar to the common redstart. The adult male is overall dark grey to black on the upperparts and with a black breast; the lower rump and tail are orange-red, with the two central tail feathers dark red-brown. The belly and undertail are either blackish-grey (western subspecies; see Taxonomy and systematics, above) or orange-red (eastern subspecies); the wings are blackish-grey with pale fringes on the secondaries forming a whitish panel (western subspecies) or all blackish (eastern subspecies). The female is grey (western subspecies) to grey-brown (eastern subspecies) overall except for the orange-red lower rump and tail, greyer than the common redstart; at any age the grey axillaries and underwing coverts are also distinctive (in the Common Redstart these are buff to orange-red). There are two distinct forms in first calendar year males at least in western subspecies, with the first ('carei') being similar to females and the second ('paradoxus') approaching adult males but lacking the whitish wing panel that does only develop during post-breeding moult of wing feathers in the second calendar year. This second form is much rarer than the first.