It is a small undershrub with thick, broadly elliptic to round leaves and white to pale pink flowers arranged singly or in twos or threes at the end of branchlets.

Philotheca rhomboidea is an undershrub that typically grows to a height of 1 m (3 ft 3 in) with glabrous, sparsely glandular-warty branchlets that become corky with age.

The leaves are thick, broadly elliptic to egg-shaped or round, 2–4 mm (0.079–0.157 in) long with two or three glandular warts on the lower surface.

[2][3][4] This philotheca was first formally described in 1970 by Paul Wilson who gave it the name Eriostemon rhomboideus and published the description in the journal Nuytsia from specimens he collected near Lake King in 1964.

[6][7] Philotheca rhomboidea grows in shrubland, often near granite or laterite, between Wongan Hills and near Esperance in the south-west of Western Australia.