Polysiphonia nigra is a species of marine alga in the division Rhodophyta.

Polysiphonia nigra is a relatively small red algae which grows to a height of 20 or 30 cm.

[1][2] The plants are dioecious with spermatrangial branchlets borne near the apices of the branches.

Tetrasporangia are arranged in short lateral branches.

[1] Generally to be found around the British Isles and in Europe from Norway to Portugal.