The beautiful nuthatch (Sitta formosa) is a bird species in the family Sittidae, collectively known as nuthatches. It is a large nuthatch, measuring 16.5 cm (6.5 in) in length, that is not sexually dimorphic. Its colour and markings are dramatic, the upper parts being black and azure, streaked with white and pale blue on the head and lined with the same colours on the wing feathers. The underparts are orange, and the eyebrow and throat are ochre. An irregular, dark eyestripe highlights its eye. Its ecology is not fully described, but it is known to feed on small insects and larvae found on the trunks and epiphyte-covered branches of trees in its range. Reproduction takes place from April to May; the nest is placed in the hole of an oak, rhododendron, or other large tree. The nest is made of plant material and fur in which the bird typically lays four to six eggs.

Although the species is found in most of the countries making up the mainland of Southeast Asia, it appears to be rare throughout its range, its population being highly local where it is found. The bird nests predominantly in montane forest at an altitudinal range from 950 m (3,120 ft) up to nearly 2,300 m (7,500 ft), with some seasonal height adjustment, down to around 300 m (980 ft) in winter. Its localised occurrence within its range makes rigorous estimates of its population difficult, but its habitat is threatened by deforestation and the species appears to be in decline. It has been classified as vulnerable by the International Union for Conservation of Nature.

The nuthatches constitute a genus Sitta of small passerine birds in the family Sittidae, typified by short, compressed wings and short, square 12-feathered tails, a compact body, longish pointed bills, strong toes with long claws, and behaviourally, by their unique head-first manner of descending tree trunks. Most nuthatches have grey or bluish upperparts and a black eyestripe. Sitta is derived from the Ancient Greek name for nuthatches, σίττη, sittē. "Nuthatch", first recorded in 1350, is derived from "nut" and a word probably related to "hack", since these birds hack at nuts they have wedged into crevices. The genus may be further divided into seven subgenera, of which the beautiful nuthatch is placed alone in Callisitta (Bonaparte, 1850), and the species was therefore sometimes called Callisitta formosa.

The beautiful nuthatch was first described in 1843 by British zoologist Edward Blyth, from a specimen he examined in Darjeeling. Its kinship with other members of the genus is unclear. The bright blue colour of its plumage invites a comparison to the blue nuthatch (S. azurea), or other blue-tinted nuthatch species such as the velvet-fronted nuthatch (S. frontalis), yellow-billed nuthatch (S. solangiae) and the sulphur-billed nuthatch (S. oenochlamys), but its distribution being focused in the eastern Himalayas, and the uniqueness of its plumage, argues against the assumption. No subspecies have been identified.

In 2014, Eric Pasquet, et al. published a phylogeny based on examination of nuclear and mitochondrial DNA of 21 nuthatch species. The position of the beautiful nuthatch within the genus was not established with certainty, having a far lower statistical association than others in the model. Nevertheless, under the findings the species appears closest evolutionarily to three clades of nuthatches: two nuthatches that prefer rocky environments, the western rock nuthatch (S. neumayer) and the eastern rock nuthatch (S. tephronota); species in the "europaea" group, including the Eurasian nuthatch (S. europaea), Siberian nuthatch (S. arctica), chestnut-vented nuthatch (S. nagaensis), Kashmir nuthatch (S. cashmirensis), Indian nuthatch (S. castanea), chestnut-bellied nuthatch (S. cinnamoventris) and the Burmese nuthatch (S. neglecta); as well as the white-tailed nuthatch (S. himalayensis), and therefore, the white-browed nuthatch (S. victoriae). These close relatives are generally all species that plaster the entrance to their nest with mud.

Described by Erik Matthysen in his 1998 treatise The Nuthatches as a bird that "deserves its name", the beautiful nuthatch has highly distinctive plumage. Its upperparts are black and azure, and it is orange on the underparts. The crown and upper mantle are black, streaked with pale blue and white. The scapulars, back and rump are an azure blue. The greater and medium coverts are black, finely edged with white, forming two narrow wing bars; the flight feathers are black and more or less lined with pale blue. The eyebrow and throat are white and buff and the eye is highlighted by an irregular, dark eyestripe. Under the wing, the white base of the primary coverts contrasts sharply with grey undertail-coverts; a distinguishing trait when viewing the bird in flight. The iris is reddish-brown or dark brown and the bill is black but for a whitish tinge at the base of the lower mandible. The lower parts are generally orange-cinnamon. The legs and feet are yellowish-brown, olive-brown or greenish-brown.

There is no sexual dimorphism. Juveniles are very similar to adults, but the streaks on the mantle are blue rather than white. The primary coverts of juveniles are also more closely lined with blue, and the underparts are paler overall, especially on the chest. Adults perform a complete moult after the breeding season, whereas juveniles only have a partial moult, in which they replace a variable number of rectrices.

The bird is large as compared with other members of the genus Sitta, measuring 16.5 cm (6.5 in) in length. The folded wing measures 98–109 mm (3.9–4.3 in) in males and 97–100 mm (3.8–3.9 in) in females. The tail is 48–60 mm (1.9–2.4 in) in males and 52–56 mm (2.0–2.2 in) in females. The beak measures between 20 mm (0.79 in) and 24.9 mm (0.98 in) and the tarsus is 19–22 mm (0.75–0.87 in) in length. The weight is not known.