The Twenty-One Balloons is a novel by William Pène du Bois, published in 1947 by the Viking Press and awarded the Newbery Medal for excellence in American children's literature in 1948.

The story is about a retired schoolteacher whose ill-fated balloon trip leads him to discover Krakatoa, an island full of great wealth and fantastic inventions.

The main story begins with the rescue of Professor William Waterman Sherman, who is picked up by a steamship while floating among a strange wreck of twenty deflated gas balloons in the North Atlantic.

Sherman's uneventful flight over the Pacific Ocean was serene until an unfortunate accident involving a seagull puncturing his balloon forced him to crash land on a small volcanic island.

He is befriended by Mr. F who informs him that he has landed on the island of Krakatoa and it is populated by twenty families sharing the wealth of a secret diamond mine - by far the richest in the world - which they operate as a cartel.

The professor concludes his speech by telling the audience he intends to build an improved balloon for a year of life in the air, financed by the diamond cufflinks he obtained in Krakatoa.

By contrast, "The Diamond as Big as the Ritz" is a parable for adults; it articulates large social themes that preoccupied Fitzgerald throughout his career as a mature writer, and which found their way into his major novels, notably The Great Gatsby.