Acacia undulifolia typically grows as an open shrub to a height of. It usually has few main branches that are erect to inclined and curved to straight. It has reddish-brown or sometimes green branchlets that are ridged and angled. Like most species of Acacia it has phyllodes rather than true leaves. They are green to grey-green or blue-green and slightly asymmetric and flat or sometimes convex or broadly elliptic in shape. The phyllodes are in length and wide and sparsely hairy or glabrous. It blooms between October and November and produces simpleinflorescences with spherical flower-heads that have a diameter of and contain 20 to 30 pale yellow flowers. The sub-glossy to blackish seed pods that form after flowering have a narrowly oblong to oblong shape and are quite straight with a length of and a width of.