| Scirpus occidentalis (S. Watson) Chase, 1904 |
| Scirpus occidentalis var. congestus Farw., 1917 |
| Scirpus ochreatus (Nees) Griseb., 1864 |
| Scirpus olneyi A. Gray, 1847 |
| Scirpus orbicephalus Beetle, 1944 |
| Scirpus pachystylus C. Wright, 1871 |
| Scirpus pacificus Britton ex Parish, 1905 |
| Scirpus pallidus (Britton) Fernald, 1906 |
| Scirpus paludosus A. Nelson, 1899 |
| Scirpus paradoxus (Spreng.) Boeckeler, 1870 |
| Scirpus parvulus Roem. & Schult., 1817 |
| Scirpus pendulus Muhl., 1813 |
| Scirpus plantagineus Sw., 1797 |
| Scirpus plicarhachis Griseb., 1866 |
| Scirpus potosiensis C.B. Clarke, 1908 |
| Scirpus potosinus Britton, 1892 |
| Scirpus pringlei Britton, 1888 |
| Scirpus pungens Vahl, 1806 |
| Scirpus pungens var. pungens Vahl, 1806 |
| Scirpus pycnostachyus Salzm. ex Steud., 1855 |