The Atomic Flounder (Born June 3, 1936, age 75) is a retired villain, from Mermaid Man and Barnacle Boy II, who can blast people, with his atomic breath. He fought Mermaid Man and Barnacle Boy. In The The Bad Guy Club for Villains, he resembles Harold.
In The Bad Guy Club for Villains, he has a shark fin, 2 fish hands and legs, a human hand, 2 alien eyes, and has a shark tail. In Mermaid Man and Barnacle Boy II, he is gray, has a green shirt with checkered pants and a black hat on his head.
- The Atomic Flounder's atomic ray, that was seen in Mermaid Man and Barnacle Boy II, is possibly a reference to Godzilla. The young picture of The Atomic Flounder was taken in 1957.
- He was actually a fish in a lab experiment but it went waywire and he became the villan he is now.