Polysiphonia opaca (C.Agardh) Moris et De Notaris is a small marine alga in the division Rhodophyta.

This small seaweed consists of tufts of branches no more than 6 cm long.

[1] The tufts are of erect branches each consisting a series of axial cells surrounded by about 20 periaxial cells all of the same forming a "collar" around the axis.

[1] This alga may be confused with Lophosiphonia reptabunda, this alga has however a turf-like habit and is less than 3 cm high unlike P. opaca which grows to over 5 cm high.

[2] Europe: British Isles to Spain, West Indies and E.Australia.