Polysiphonia fibrillosa (Dillwyn) Sprengel is a species of marine red alga in the Rhodophyta.

[1] Polysiphonia fibrillose is a fine red alga which grows to 25 cm long.

[2] Other similar species include: P. rhunensis Thuret et Bornet, P.orthocarpa Rosenvinge.

Cystocarps are ovoid or globular and tetrasporangia are formed in series near the apices.

Also grows epiphytically,[3] The species is widespread around Great Britain, including Shetland, and Ireland.