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"
Mermaid Man accidentally leaves his belt when he and Barnacle Boy flee from SpongeBob. Though SpongeBob intially plans to return it, he decides to take the belt and plays with it, which he discovers the shrink ray button. He makes his own little cockroach tea party while still working by shrinking the Krusty Krab kitchen appliances and food and feeding it to the cockroach.
Squidward discovers that SpongeBob took the belt and decides to call Mermaid Man and Barnacle Boy as revenge for making the muscular fish punch him. SpongeBob begs Squidward not to tell them but Squidward does not listen so he shrinks Squidward. Squidward, obviously, gets angry at SpongeBob for shrinking him and tells SpongeBob to un-shrink him but SpongeBob is unsuccessful. Squidward suggests asking for help from Mermaid Man and Barnacle Boy but SpongeBob refuses, preferring to look for someone "wise" instead, and ultimately resorts to asking for help from Patrick.
Patrick flips the M to W and SpongeBob zaps Squidward thinking it would un-shrink only for Patrick to be shrunken as well. Squidward tells SpongeBob to go to Mermaid Man but SpongeBob still does not want to ask for help from him; he keeps them in a jar. Suddenly, Sandy appears, prompting SpongeBob to shrink her. Soon after, Larry and a few Bikini Bottomites appear, prompting SpongeBob to shrink them as well. SpongeBob then makes a run for it, shrinking every Bikini Bottomite he encounters until everyone is small. SpongeBob places the shrunken people in the jar. It turns out SpongeBob shrank Mermaid Man and Barnacle Boy as well. When the people tell Mermaid Man to tell SpongeBob to un-shrink them, Mermaid Man thinks for a while and asks SpongeBob if he set it to Wumbo, which makes everyone realize that he does not know how to un-shrink them either.
The Bottomites burst out of the jar and enter SpongeBob's body to attack his body parts as retaliation for shrinking them. Ultimately, SpongeBob shrinks the entire city before shrinking himself. The citizens are happy with this since there's no difference, which they celebrate and SpongeBob is forgiven. A few moments later, a bus drops off Plankton, returning to Bikini Bottom from vacation, surprised to see that Bikini Bottom is smaller than him.