Key to the species of Polysiphonia
| 1 | Pericentral cells 4 | 2 |
| 1 | Pericentral cells 5 or more | 7 |
| 2 | Plants growing in brackish water or nearshore in calm places; with lateral branches arising independent of trichoblasts; trichoblasts poorly developed, infrequent | P. subtilissima Montagne |
| 2 | Plants not growing in brackish water; lateral branches formed in place of trichoblasts in fixed pattern | 3 |
| 3 | Plants having nearly equal prostrate and erect portions | 4 |
| 3 | Plants having mostly erect portions | 5 |
| 4 | Rhizoids continuous with pericentral cells; erect axes mostly unbranched; tetrasporangia in straight series | P. scopulorum Harvey |
| 4 | Rhizoids large, saccate, pit-connected to nearly every pericentral cell; epi-endophytes of Codium species | P. saccorhiza (Collins & Hervey) Hollenberg |
| 5 | Rhizoids cut off from proximal end of pericentral cells; trichoblasts delicate, deciduous; tetrasporangia in slightly spiral series | P. upolensis (Grunow) Hollenberg |
| 5 | Rhizoids cut off from middle part of pericentral cells; trichoblasts persistent | 6 |
| 6 | Plants reddish-brown, silky, branching subdichotomous, trichoblasts on every segment in 1/4 spiral arrangement; branches following spiral pattern; plants growing on various algae | P. sphaerocarpa Børgesen |
| 6 | Plants nearly black, coarse; branching irregularly pinnate; trichoblasts on every segment, but laterals formed independently of trichoblasts every 5-6 segments; on hard substrates in turf | P. hancockii Dawson |
| 7 | Plants with axes to 700 µm in diam.; laterals cigar-shaped, prominently constricted at their bases; pericentral cells 4-5(6) | P. beaudettei Dawson |
| 7 | Plants with axes 175-275 µm in diam., laterals not cigar-shaped, wide-angled; pericentral cells 5 | P. pentamera Hollenberg |