Posidonia robertsoniae is one of the seagrasses of Western Australia, submerged flowering plants that occur in the southern coastal waters.

This species is found at depths from 0.5 to 20 metres on white sands, in coastal waters that may be sheltered or subject to intense wave action.

The habit of Posidonia robertsoniae is as a clumping stand, it is rhizomatous plant that puts out shoots with one or two leaves.

Posidonia robertsoniae is recorded at coasts of Southwest Australia, from Warnbro Sound to Israelite Bay.

The species Posidonia coriacea, which occurs in a wider distribution range, and this one may not be distinct, both lack evidence of genetic variation.