This article is about the real-life under-sea organisms. For the character in the show SpongeBob SquarePants, see Plankton (SpongeBob SquarePants)

Plankton are any drifting organism that inhabits the pelagic zone of oceans, seas, or bodies of fresh water. It is a description of life-style rather than a genetic classification. They are widely considered to be some of the most important organisms on Earth, due to the food supply they provide to most aquatic life.