Arctopus echinatus is a stemless, summer-deciduous perennial up to 6 cm high and 60 cm in diameter. The roots are relatively large and tuberous and exude a sticky resin when broken. The leaves are large, simple, prostrate and ovate to rhomboidal in outline with spiny margins and sharp recurved thorns between the leaf divisions. The plants are either male or female. Male flowers are cream and borne on short flower stalks while female flowers are greenish and stalkless and arranged in flower-like heads (pseudanthia) surrounded by four or five prominent spiny bracts. The fruit is dry, brown and spiny. Flowering occurs from May to July.

The genus Arctopus is an unusual member of the family Apiaceae with each plant being either male or female (i.e. dioecious). Male and female plants are quite unmistakable with the males bearing umbels of small cream flowers and the females bearing greenish flowers enclosed by large spiny bracts to form a flower-like head, or pseudanthium. Pseudanthia are commonly found in other closely related and often ornamental genera, such as Alepidea, Astrantia and Eryngium.

The shape, size and spinescence of these pseudanthium bracts are important diagnostic characters to distinguish between the three species (Magee et al. 2008). In Arctopus echinatus they are boat-shaped, with an acute spine-tipped apex and inflexed, overlapping spines along the margin.

In the threatened A. dregei Sond. they are rolled inwards, with an obtuse spine-tipped apex and inflexed, spinescent hairs along the margin, while in A. monacanthus Carmich. ex Sond. they become very large and foliose, with an obtuse, spine-tipped apex and a usually entire margin.