The Atomic Flounder 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. He also appeared in the Yellow Avenger video game as the picture on the right in the Industrial Park.
In The Bad Guy Club for Villains, he has a shark fin, 2 fish hands and legs, a human hands, 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.