Shag Rock is a small rocky island in the Wallabi Group of the Houtman Abrolhos.

This platform, which arises abruptly from a flat shelf, is about 40 m (131 ft) thick, and is of Quaternary origin.

Reef that formed during the Eemian Stage (about 125,000 years ago), when sea levels were higher than at present, are now emergent in places, and constitute the basement of the group's "central platform" islands, of which Shag Island is one.

A very small area just south of the centre is vegetated by Tecticornia halocnemoides (Shrubby Samphire).

The remainder of the island, predominantly the northern third and a patch in the south west, is vegetated by Atriplex cinerea (Grey Saltbush), Nitraria billardierei (Nitre Bush) and Pittosporum phillyreoides (Weeping Pittosporum).