The Sinister Slug is an enemy of Mermaid Man and Barnacle Boy. He appears in the episodes "Mermaid Man and Barnacle Boy II," "The Bad Guy Club for Villains," and "Appointment TV," as well as the video games The Yellow Avenger, SpongeBob's Truth or Square, SpongeBob SquigglePants, and SpongeBob Moves In!


In "Mermaid Man and Barnacle Boy II" and the video games, he is a gray sea slug with red eyes on stalks, a pair of thick, black eyebrows, and a mouth filled with sharp teeth. He wears a green mask and a green suit with a purple collar and a golden triangle on it.

From "The Bad Guy Club for Villains" onwards, he is more realistically designed, and now has a beige body with brown spots and hair sprouting in several places. His teeth are more sharp and jagged, and his eyes are fully red.


The Sinister Slug can use the slime he produces to climb up walls, and can spit slime onto his enemies to trap them.


"Mermaid Man and Barnacle Boy II"

He appears as a cardboard cutout during The Adventures of Mermaid Man and Barnacle Boy.

"The Bad Guy Club for Villains"

In the lost episode of "The Adventures of Mermaid Man and Barnacle Boy," he was one of the villains to join the B.G.A.T.F.B.C. During the fight between the heroes and the villains, he tries to attack Mermaid Man with his slime, but Mermaid Man dodges the slime and causes the Atomic Flounder to get hit instead. At the lost episode's end, he and the other villains reveal that they were actually together for a book club.

"Appointment TV"

He appears on a poster for the "Mermaid Man and Barnacle Boy" episode The Case of the Curious Cliff Hanger.

The Yellow Avenger

After the Yellow Avenger defeats Man Ray, Squidward reveals that the Sinister Slug has traveled to Jellyfish Fields to cause trouble and prevent the talent show from happening. The Sinister Slug managed to scare Squidward away and made him drop his clarinet while doing so, and Squilliam Fancyson was scared so much that he left for home.

After hearing about the Sinister Slug scaring Squilliam, the Yellow Avenger travels to Jellyfish Fields and uses his strength superpower to get up to where the slug is. After confronting him, the Sinister Slug admits that he can't fight the power that Mermaid Man's belt has, and reluctantly leaves without battling the Yellow Avenger.

SpongeBob's Truth or Square

A cardboard cut-out of him appears in the DS version during the "Mermalair" minigame, where it can be hit with Waterballs.

SpongeBob SquigglePants

He appears as a cardboard cutout that the player fights in one of the nano-games.

SpongeBob Moves In!

He appears as a cardboard cutout that the player can buy and place in Bikini Bottom.

SpongeBob Comics Annual Super-Giant Swimtacular No. 1

He appears in the book's bonus poster, along with many other Mermaid Man villains, and a wax figure of him is featured in the comic The Secrets of the Merma-Lair!

SpongeBob Freestyle Funnies 2018

He appears on the comic book's cover staring angrily at SpongeBob, along with many other villains.



Title Role(s) in episode
Season 1
"Mermaid Man and Barnacle Boy II" (tr) Debut; cameo as cardboard cutout
Season 7
"The Bad Guy Club for Villains" (tr) Major
Season 11
"Appointment TV" (tr) Cameo on poster


Title Role(s) in game
The Yellow Avenger Minor antagonist
SpongeBob's Truth or Square Cardboard cut-out; seen in the DS version
SpongeBob SquigglePants Cameo as cardboard cut-out
SpongeBob Moves In! Cameo as cardboard cut-out decoration
Mermaid Man and Barnacle Boy (VE)
SpongeBob's Truth or Square (VE)

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