Polysiphonia elongata is a small red alga which, unlike some other species of Polysiphonia, does not grow as tufts. It has erect cylindrical main branches and is densely branchedgrowing to a height of 30 cm. The main branches bear lateral branches, each branch consists of a central axes of cells with four periaxial cells. These periaxial cells are arranged in an elongatedmanner all of the same length around the central axial cells. Cortical branches develop growing downwards in the grooves between the periaxial cells and increasing the thickness of the major branch. The final branches are pointed at the apex and attenuate at their base. Trichoblasts are few. It grows attached by a disk-like holdfast.