The Mad King is a Ruritanian romance by American writer Edgar Rice Burroughs, originally published in two parts as "The Mad King" and "Barney Custer of Beatrice" in All-Story Weekly, in 1914 and 1915, respectively. These were combined for the book edition, first published in hardcover by A. C. McClurg in 1926.

Set in the fictional European kingdom of Lutha, the protagonist is a young American named Barney Custer, of Beatrice, Nebraska, who is the son of an American farmer and a runaway Luthan princess, Victoria Rubinroth. Unaware of his royal blood, much less that he is a dead ringer for his relative Leopold, the current king of Lutha, Barney visits Lutha on the eve of the First World War to see for himself his mother's native land. As he arrives in Lutha, King Leopold has just escaped from his ten years' imprisonment at the hands of his scheming uncle, Prince Peter of Blentz. Much to his own and everyone else's confusion, Barney is naturally mistaken for the king, leading to numerous complications.

Barney meets and falls in love with Princess Emma Von Der Tann, Leopold's promised bride and then becomes intimately involved in Luthan affairs, working to help the king and ultimately allowing himself to be proclaimed as king while impersonating Leopold to prevent Prince Peter from seizing the throne. He finally succeeds in foiling Peter's plans to become king himself by rescuing and fighting for the real king. Unfortunately, after his coronation, King Leopold discovers the shared love between Barney and Princess Emma, and Barney is forced to leave Lutha, mimicking the flight of his father years earlier, though his father left with a princess—Barney has only a soldier. Thus ends part one.