Mad Hatter may refer to:

• The Hatter, a fictional character in Alice's Adventures in Wonderland
• Mad hatter disease

The Hatter (also called the Mad Hatter or Hatta) is a fictional character in Lewis Carroll's Alice's Adventures in Wonderland and its sequel Through the Looking-Glass. He is often referred to as the Mad Hatter, though this term was never used by Carroll. The phrase "mad as a hatter" pre-dates Carroll's works and the characters the Hatter and the March Hare are initially referred to as "both mad by the Cheshire Cat, with both first appearing in Alice's Adventures in Wonderland, in the seventh chapter titled "A Mad Tea-Party".