Cleidothaerus albidus is a bivalve mollusc of the family Cleidothaeridae, the only member of its genus and family. It is endemic to south east Australia and the North Island of New Zealand including the Chatham Islands.
The shell is large and very solid, cemented to the rock by its right valve, which has the appearance of a swallow's nest. The left valve is flat, spirally coiled at the umbo, or beak, and from there to the ventral margin there is a median sulcus.
Coloration of the exterior is yellowish-olive, with an interior dull pearly, often tinged with pale pink and olive-green.
Height is up to 47 mm, width 68 mm.