Craven & R.D.Edwards Calothamnus quadrifidus, commonly known as one-sided bottlebrush,[2] is a plant in the myrtle family, Myrtaceae and is endemic to the south-west of Western Australia.

Widely cultivated because of its attractive foliage, colourful, unusual and prolific flowers, it grows in a variety of habitats and soils.

[4][5] Calothamnus quadrifidus was first formally described in 1812 by Robert Brown from a specimen he collected at Lucky Bay near Esperance during the Investigator expedition with Matthew Flinders.

[9] Calothamnus quadrifidus is classified as "not threatened" by the Western Australian Government Department of Parks and Wildlife[8] but subspecies asper is listed as "Priority Two"[10] meaning that it is poorly known and from only one or a few locations.

Subspecies teretifolius is listed as "Priority Four" by the Government of Western Australia Department of Parks and Wildlife,[11] meaning that is rare or near threatened.