The goldcrest is the smallest European bird, 8.5–9.5 cm (3.3–3.7 in) in length, with a 13.5–15.5 cm (5.3–6.1 in) wingspan and a weight of 4.5–7.0 g (0.16–0.25 oz). It is similar in appearance to a warbler, with olive-green upper-parts, buff-white underparts, two white wing bars, and a plain face with conspicuous black irises. The crown of the head has black sides and a narrow black front, and a bright crest, yellow with an orange centre in the male, and entirely yellow in the female; the crest is erected in display, making the distinctive orange stripe of the male much more conspicuous. The small, thin bill is black, and the legs are dark flesh-brown. Apart from the crest colour, the sexes are alike, although in fresh plumage, the female may have very slightly paler upper-parts and greyer underparts than the adult male. The juvenile is similar to the adult, but has duller upper-parts and lacks the coloured crown. Although the tail and flight feathers may be retained into the first winter, by then the young birds are almost indistinguishable from adults in the field. The flight is distinctive; it consists of whirring wing-beats with occasional sudden changes of direction. Shorter flights while feeding are a mix of dashing and fluttering with frequent hovering. It moves restlessly among foliage, regularly creeping on branches and up and down trunks.
The nominate subspecies, R. r. regulus
, in Belgium. The goldcrest has a bright crest and a relatively plain face. The orange tinge of the hindcrown indicates that this is a male.
The goldcrest is usually easily distinguished from other small birds in its range, but poor views could possibly lead to confusion with the common firecrest or yellow-browed warbler. The adult common firecrest has a distinguishing face pattern showing a bright white supercilium (eyebrow) and black eye-stripe, and the juvenile usually shows enough of this face pattern to be readily distinguished from the plain-faced goldcrest. The yellow-browed warbler has a yellowish supercilium and pale crown stripe, so also shows a different head pattern. The ruby-crowned kinglet, an American Regulus species and a potential vagrant in Europe, could be more difficult to distinguish. It has a plain face like its Old World cousin, but the male has a red crest without any yellow or a black border. Female and juvenile ruby-crowned kinglets lack the ruby-red crown patch, but compared with the similarly crestless juvenile goldcrest, the American bird is larger in size, has an obvious whitish eyering, and yellowish wing bars.