Scytosiphon lomentaria is a littoral brown seaweed with an irregularly lobed many filamentous form. It is a member of the Phaeophyta in the order Dictyosiphonales and grows attached to shells and stones in rock-pools and in near-shore waters. The attachment to the substrate is by a small disc shaped holdfast.

Scytosiphon lomentaria has cylindrical, shiny, olive brown, unbranched fronds up to 400 mm long. They have short stalks and a large number may arise from a single holdfast. They widen to 3-10mm and narrow again near the tip. They are hollow and often have irregular constrictions.^[1]

This species is cosmopolitan in distribution being found in temperate waters around the world. The type location is Denmark.^[2] It occurs in the littoral zone and favours wave-exposed shores and rock pools.^[3] Small plants are often found growing on limpets and pebbles.^[1]