Mad as a hatter is a colloquial English phrase suggesting insanity.

Mad as a hatter may also refer to:

• "Mad as a Hatter", an episode of Batman: The Animated Series
• "Mad as a Hatter", an episode of The Real Housewives of Dallas (season 1)
• "Mad as a Hatter", an episode of Ladies of London
• Mad As A Hatter (EP), by Def Wish Cast, 1993, and a track on the EP
• Mad as a Hatter, an album by Shadowland, 1996
• "Mad as a hatter", part of mnemonic for features of anticholinergic syndrome

• Mad Hatter (disambiguation)
• Hatter (disambiguation)
• Erethism, or mad hatter disease
• Mad as a March hare