Mermaid Man's Belt is an item owned and worn by Mermaid Man.
From a distance, Mermaid Man's Belt looks like an ordinary belt that is violet in color with the letter "M" that is metallic blue in color (in Mermaid Man's early appearances, it is yellow), but up close, it appears to have a bunch of hardware inside of it.
In the episode, "Mermaid Man and Barnacle Boy IV," SpongeBob accidentally 'steals' Mermaid Man's belt when he left it behind from running away from their freakishly eager fan, SpongeBob SquarePants. SpongeBob discovers the Shrinkray button, and he makes his own little bug tea party while still working by shrinking the Krusty Krab tables and food and feeding it to the bugs. He even shrinks his hat. Squidward finds him, and hence, threatens to call Mermaid Man and Barnacle Boy. SpongeBob gets upset by this and got so out of control with crying that he accidentally zapped Squidward with the Shrinkray. He tries to find a "wise" friend and quickly resorts to Patrick.
Patrick mistakes the new mini Squidward for an action figure and unstoppably starts to play childishly with him. SpongeBob accidentally zaps Patrick with the Shrinkray too, and then Sandy, then Larry, followed by everyone in Bikini Bottom. Everyone, except for Mermaid Man and Barnacle Boy, 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, then himself, and everyone is 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.
According to SpongeBob, Mermaid Man has worn the belt for 65 years.