From a distance, Mermaid Man's belt looks like an ordinary belt that is violet purple with the letter "M" that is light metallic blue, but up close, it appears to have a bunch of hardware inside of it.
In Mermaid Man's earlier appearances, it is yellow, similar to Aquaman's belt.
Role in series
"Mermaid Man and Barnacle Boy IV"
SpongeBob accidentally "steals" Mermaid Man's belt when he left it behind. SpongeBob decides to wear the belt and discovers the shrink ray button. He makes his own little bug tea party while still working by shrinking the Krusty Krab tables and food and feeding it to the bug.
Squidward finds him after getting disrupted by his work, and hence, threatens to call Mermaid Man and Barnacle Boy after he discovers that SpongeBob has the belt. SpongeBob gets upset over this and gets so out of control with crying that he accidentally zaps Squidward with the shrink ray. Worried that Mermaid Man will find out that he has his belt, he tries to find a "wise" friend and quickly resorts to Patrick.
After Patrick suggests to flip the M upside down, SpongeBob accidentally zaps Patrick with the shrink ray too, and then Sandy, then Larry, followed by everyone in Bikini Bottom. Everyone, except for Mermaid Man, Barnacle Boy, and Patrick, is mad at SpongeBob and start to attack his organs inside his holes. Then suddenly, SpongeBob for once comes up with a bright idea: to shrink Bikini Bottom, and so he does, making himself and everyone happy.
At the end of the episode, the bus arrives in large size and drops off Plankton, and humorously, he is confused with why the town is now just the right size for him, being small.