The Atomic Flounder is a retired villain of Mermaid Man and Barnacle Boy. He is seen in the episode Mother Goose B. Goode. In the episode, Norman points him out to Mother Goose, and tells him that "he doesn't look like much, but he could go back to crime...just like that." That was enough for Mother Goose to attempt to arrest him. This enraged the Atomic Flounder enough to attack Wilbur with his atomic breath.
The Atomic Flounder seems to be the only villain that aged unlike ManRay, the Dirty Bubble, The Moth, the Sinister Slug, the Jumbo Shrimp, and Kelp Thing. He is a very light colored flounder. He wears a green sweater over a yellow shirt and red tie, brown plaid pants, a tiny hat and carries an umbrella as cane, but in The Bad Guy Club for Villains (Episode) he was lavender and his eyes are like aliens and he has a tail and he has no sweater, no umbrella, no plaid pants, no tie, no hat and no shirt, just a thing used for box fights.
- The Atomic Flounder's atomic ray was possibly taken from Godzilla.
- He may be related to Flats and/or Brian Flounder.The Atomic Flounder is also about 73
- "If I weren't retired I'd WRRRRRRROOOOOOOOOONNNNNNNNNGGGGGGG!!!!!!!!!!!"