Trichodesmium, also called sea sawdust, is a genus of filamentous cyanobacteria. They are found in nutrient poor tropical and subtropical ocean waters (particularly around Australia and in the Red Sea, where they were first described by Captain Cook). Trichodesmium is a diazotroph; that is, it fixes atmospheric nitrogen into ammonium, a nutrient used by other organisms. Trichodesmium is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally. Trichodesmium is the only known diazotroph able to fix nitrogen in daylight under aerobic conditions without the use of heterocysts.

Trichodesmium can live as individual filaments, with tens to hundreds of cells strung together, or in colonies consisting of tens to hundreds of filaments clustered together. These colonies are visible to the naked eye and sometimes form blooms, which can be extensive on surface waters. These large blooms led to widespread recognition as "sea sawdust/straw"; in fact, the Red Sea gets most of its eponymous colouration from the corresponding pigment in Trichodesmium erythraeum. Colonies of Trichodesmium provide a pseudobenthic substrate for many small oceanic organisms including bacteria, diatoms, dinoflagellates, protozoa, and copepods (which are its primary predator); in this way, the genus can support complex microenvironments.