Polysiphonia foetidissima Cocks ex Bornet (also known as Vertebrata foetidissima) is small red marine alga in the Rhodophyta.

[1] Polysiphonia foetidissima grows in dense branching tufts to a length of 11 cm.

The fine erect branches have a central axis surrounded by 7 or 8 pericentral cells all of the same length.

[1] Only recorded twice from Ireland and Great Britain.

[2] Further records are from France, Portugal, the Mediterranean and Bermuda.