Bikini Bottom's populace, like that of the rest of the series, consists of various anthropomorphic fish, whales, crabs, sponges, squid, starfish, anchovies, lobsters, sharks, and most other forms of sea life, including sea monsters. Some forms of sea life, like jellyfish, seahorses, snails, and worms aren't anthropomorphic, and are treated like pets or wild animals. For example, snails represent cats, and worms represent dogs. The first known land creature to live in Bikini Bottom was the Dark Knight, an ancestor of Sandy Cheeks, before the 12th century. Sandy is the only land creature seen living in modern Bikini Bottom. Land creatures like humans, bugs and birds are scarcely seen. Inhabitants of Bikini Bottom are called "Bikini Bottomites".

The title character of the series. He is an optimistic sponge who works at the Krusty Krab, attends high school, and hangs out with his friend Patrick, or spends time with Sandy, a squirrel.

SpongeBob's best friend and neighbor, who is a starfish. He is known to not do anything, and usually gets SpongeBob mixed into his situations.

SpongeBob's neighbor and co-worker. Squidward finds SpongeBob and Patrick to be annoying. He also plays the clarinet (badly), and has a deep interest in the arts, believing that the other Bikini Bottom residents are uncultured.

SpongeBob and Squidward's boss, a money-obsessed crab who is the owner and founder of The Krusty Krab.

A female squirrel from Texas.

SpongeBob's pet snail.

Mr. Krabs' rival who is the owner and founder of the Chum Bucket. He continually tries to steal the Krabby Patty formula but has never succeeded.