The siskin is a small, short-tailed bird, 11–12.5 centimetres (4.3–4.9 in) in length with a wingspan that ranges from 20 to 23 centimetres (7.9 to 9.1 in). It weighs between 12 and 18 grams (0.42 and 0.63 oz).
The bird's appearance shows sexual dimorphism. The male has a greyish green back; yellow rump; the sides of the tail are yellow and the end is black; the wings are black with a distinctive yellow wing stripe; its breast is yellowish becoming whiter and striped towards the cloaca; it has a black bib (or chin patch) and on its head it has two yellow auriculas and a black cap. The amount of black on the bib is very variable between males and the size of the bib has been related to dominance within a flock. The plumage of the female is more olive-coloured than the male. The cap and the auriculas are greenish with a white bib and a rump that is a slightly striped whitish yellow. The young have a similar colouration to the females, with drab colours and a more subdued plumage.
The shape of the siskin's beak is determined by its feeding habits. It is strong although it is also slender in order to pick up the seeds on which they feed. The legs and feet are dark brown and the eyes are black.
It has a rapid and bounding flight pattern that is similar to other finches.
The siskin is easy to recognize, but in some instances it can be confused with other finches such as the citril finch, the European greenfinch or the European serin. The Eurasian siskin, in many plumages, is a bright bird. Adult male Eurasian siskins are bright green and yellow with a black cap, and an unstreaked throat and breast. Adult females also usually have green and yellow plumage tones: for example, yellow in the supercilium and on the sides of the breast, green tones in the mantle and yellow in the rump. The ground colour of the underparts of the Eurasian siskin is normally pure white. In females and juveniles, the centre of the belly and lower breast are often largely or entirely unstreaked. The wingbars of the Eurasian siskin are broad and yellow (with the tips white) and the bill is short with a decurved culmen.