Undaria pinnatifida is a brown seaweed that can reach an overall length of about 0.5-3 m. It is usually found in sheltered, rocky areas. It is also found on structures and boats (see picture), and is present in Tasmania and southern Victoria. It is an annual species with two separate life stages: a macroscopic diploid stage (the sporophyte, which is the ‘frond’ form), usually present through the late winter to early summer months, and a microscopic haploid stage (the gametophyte), present during the remaining months. The sporophyte is generally golden-brown in colour, with a lighter-coloured stem of varying length that extends to a distinct midrib in mature sporophytes. The fronds of up to 80 cm width are undivided in juvenile and pinnate in mature individuals.

During the reproductive season, two distinct convoluted or ’frilled’ sporophylls - one on each side of the stem, usually interleaving with one another and giving the impression of being only one piece - are found at the base of the stipe and above the holdfast. The midrib and sporophylls are key identification features of this alga. In areas with wide temperature ranges and maxima approaching 30 °C in summer, the sporophyte is only visible during the cooler months, while gametophytes are also present in the warmer months. In areas with lower temperature variability and an upper temperature limit of no more than 19 °C, sporophytes and gametophytes may be present throughout the whole year.

Undaria pinnatifida is listed on the Australian Priority Marine Pest List and should be reported to authorities if found outside of its current known distribution.

The species was first reported from eastern Tasmania in 1988, but may have been present since 1982. It was reported in Port Phillip Bay, Victoria, in 1996. Molecular studies have demonstrated that this was a separate introduction from the one in Tasmania.