Posidonia ostenfeldii is a species of seagrass that occurs in the southern waters of Australia.

A perennial rhizomatous herb that appears as clumps on sand in marine habitat.

Posidonia ostenfeldii is recorded at the edge of the Esperance Plains, the Archipelago of the Recherche, at the southern coast of Southwest Australia.

This species is contained by the Posidoniaceae family, one of eight occurring in southern Australia.

The ninth member, Posidonia oceanica, is found in the mediterranean sea.