Spirogyra (common names include water silk, mermaid's tresses, and blanket weed) is a genus of filamentous charophyte green algae of the order Zygnematales, named for the helical or spiral arrangement of the chloroplasts that is characteristic of the genus. Spirogyra species, of which there are more than 500, are commonly found in freshwater habitats. Spirogyra measures approximately 10 to 150 micrometres in width (though not usually more than 60) and may grow to several centimetres in length.

Spirogyra can be found on every continent on Earth, including Antarctica. It is freshwater algae, found in rivers, ponds, and other such bodies of water.

The genus Spirogyra was named by German naturalist Johann Heinrich Friedrich Link in 1820. The lectotype, Spirogyra porticalis was designated in 1952 by Paul C. Silvia.

Spirogyra can reproduce both sexually and asexually. In vegetative reproduction, fragmentation takes place, and Spirogyra simply undergoes intercalary cell division to extend the length of the new filaments.

Sexual reproduction is of two types:

The essential difference is that scalariform conjugation occurs between two filaments and lateral conjugation occurs between two adjacent cells on the same filament.

Spirogyra species are being researched for their potential in bioremediation. Specifically, in stemming toxic runoff from mines (where they are often found occurring naturally) and from municipal wastewater. Spirogyra has also been investigated as a potential biofuel.

Spirogyra species, such as S. varians, are also being researched for potential pharmaceutical usage due to their high nutrient densities.