Chordaria flagelliformis is a brown macroalga (seaweed) in the family Chordariaceae, phylum Ochrophyta, found in cool to cold marine waters of the Northern Hemisphere, including the coasts of northern Europe (where it is recorded from Norway, the British Isles, and the Baltic region), northern North America, and parts of the northern Pacific. It is an intertidal and shallow subtidal species, typically growing in low to mid intertidal zones attached to rocks and shells by a discoid holdfast, producing long, cylindrical, rope-like or whip-like fronds — the specific epithet flagelliformis means whip-shaped — that may reach 30–60 centimetres in length. The thallus is slimy in texture due to mucilage production and is olive-brown to greenish in colour. Chordaria exhibits a heteromorphic alternation of generations — the conspicuous macroscopic generation is the sporophyte, while the microscopic generation is the filamentous gametophyte stage. The species contributes to the productivity of coastal rocky intertidal and subtidal communities and provides attachment substrate and shelter for small invertebrates and epiphytic organisms. The IUCN classifies this seaweed as Least Concern. It is a common component of Northern European rocky shore communities and is not threatened by harvesting or other human activities.