Watsonia angusta grows up to 2 m high. It has an underground, depressed globose corm 20–30 mm in diameter, with a coarsely reticulate, grey-brown tunic; sometimes several cormlets are found in the axil of the lower leaves. The stem is simple or usually with a few, short, diverging branches.

Leaves are 5 to several, more or less basal, but the uppermost of these sheath the stem, and are about half as long as the plant. They are narrowly lanceolate and sword-shaped;   the margins are slightly to moderately thickened and hyaline.  The leaves dry off and the plant is dormant during the dry season.

The inflorescence is a lax, up to 18-flowered spike. Flowers are red to orange; it has a long cylindric upper perianth tube with very narrow petals 4–8 mm wide, with a somewhat hooded upper tepal, reflexed lateral tepals and a strongly recurved lower tepal. The fruit is an up to 35 mm long, tapering, fusiform capsule, splitting open to release small, linear seeds. It flowers from November to January.