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The smallest member of the genus Malurus, the red-backed fairywren measures 11.5 cm (4 1⁄2 in) and weighs between 5–10 g (0.18–0.35 oz), averaging around 8 g (0.28 oz). The 6 cm (2 1⁄4 in) long tail is black in the breeding male, and brown in eclipse males, females and juvenile birds. Averaging 8.6 mm (0.3 in), the bill is relatively long, narrow, pointed and wider at the base.Wider than it is deep, the bill is similar in shape to those of other birds that feed by probing or picking insects off of their environs.

Like other fairywrens, the red-backed fairywren is notable for its marked sexual dimorphism; the male adopts full breeding plumage by the fourth year, later than all other fairywrens apart from the closely related white-winged fairywren. The male in breeding plumage has a black head and body with striking red back and brown wings. At other times it has a brown upper body and white underparts. Some males, mainly younger, remain in eclipse plumage while breeding.[The female looks remarkably similar with a buff brown body and a yellowish spot under the eye. The female of this species differs from those of other fairywren species in that it lacks a blue tint in the tail. Geographically, it follows Gloger's rule; female birds have whiter bellies and paler brown upperparts inland in sunnier climates. Juveniles of both sexes look very similar to females.