The Nostocaceae are a family of cyanobacteria that forms filament-shaped colonies enclosed in mucus or a gelatinous sheath.

Most benthic algae of the order Nostocales belong to this family.

Like other cyanobacteria, these bacteria sometimes contain photosynthetic pigments in their cytoplasm to perform photosynthesis.

Species of the Nostocaceae are particularly known for their nitrogen-fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern, cycads, and hornworts.

They and the nitrogen they contain are then plowed into the soil following the harvest, which has proved to be an effective natural fertilizer.