Plankton (real name Sheldon J Plankton) is a fictional character in the animated series SpongeBob SquarePants. He is a technologically savvy, deep-voiced zooplankter with one central eye—a real-life characteristic of some copepods. He is the smallest of the main characters in the series, able to fit in SpongeBob SquarePants's hand, as well as under Mr. Krabs's foot.

Plankton is Mr. Krabs's business rival. He runs a competing restaurant called The Chum Bucket , where no one eats, located directly across the street from The Krusty Krab. Plankton has a number of plans to put Eugene Krabs out of business, all of which involve getting a hold of the secret recipe for the "Krabby Patty" served at The Krusty Krab. Plankton is about the same age as Mr. Krabs. Plankton lives at The Chum Bucket with Karen, his computer "wife".

As disclosed in the episode "Plankton's Army", Plankton's first name is Sheldon, which amuses Karen to no end. In the same episode, it is revealed that Plankton has been trying to find out Krabs's secret recipe for twenty-five years.