Ceratophyllum is a genus of flowering plant, commonly found in ponds, marshes, and quiet streams. They are usually called hornworts, although this name is also used for unrelated plants of the division Anthocerotophyta. Ceratophyllum grows completely submerged, usually though not always floating on the surface. At intervals along the stem they produce rings of leaves, which are narrow and often branched. Because of their appearance and their high oxygen production, they are often used in freshwater aquaria.
Ceratophyllum is given its own family, the Ceratophyllaceae, and in newer systems its own order, the Ceratophyllales. It belongs among the basal dicotyledons, and may be the closest relative of the monocotyledons. The division into species is not completely settled; C. demersum is particularly notable.