Posidonia australis, also known as fibre-ball weed or ribbon weed, is a species of seagrass that occurs in the southern waters of Australia. It forms large meadows important to environmental conservation. Balls of decomposing detritus from the foliage are found along nearby shore-lines. In 2022, a single stand in Shark Bay was reported by scientists to not only be the largest plant in the world, but the largest organism by square size.

Posidonia australis is a flowering plant occurring in dense meadows, or along channels, in white sand. It is found at depths from 1–15 m (3–49 ft). Subsurface rhizomes and roots provide stability in the sands it occupies. Erect rhizomes and leaves reduce the accumulation of silt. The leaves are ribbon-like and 11–20 mm (0.43–0.79 in) wide. They are bright green, perhaps becoming browned with age.