Acacia angustissima grows as a thornless shrub or small tree mostly 2-7 m tall with a single short trunk. It exhibits much variation in pubescence, size and venation of the leaflets and size of flowers and heads.
The leaves are mostly asymmetric with a displaced mid-vein, 10-25 cm long, with 10-20 pairs of pinnae and leaflets without secondary venation.
The inflorescences are ellipsoidal with whitish heads 1-1.5 cm in diameter, turning pinkish to dull orange when dried.
The pod is oblong, 3-6 cm long and 6-9 mm wide, with straight or sinuate margins. The pods are initially green, turning coffee-brown as they ripen.