Madman's Island is a 1927 novel by Ion Idriess set in northern Australia.

It was Idriess' first novel and was semi-autobiographical, although he invented the love interest at the insistence of the publisher.

Jack Burnett decides to go prospecting on an uninhabited island in the Barrier Reef with a friend. The friend goes mad and tries to kill Jack. Jack discovers some opium stashed away by Japanese smugglers. Jack is rescued and sells the opium to a Chinese merchant in Cooktown.

In Cairns, Jack runs into the Japanese smugglers, but manages to escape with the help of a woman he has fallen in love with.

The book was based on a true incident that happened to Idriess. In 1923 he was marooned on Howick Island in Queensland with a friend he had gone prospecting with. The friend had a war injury which sent him mad and he tried to kill Idriess.

Idriess kept a diary of his time on the island and used it as the basis for the book. He sold it to a publisher in 1925.

Idriess fictionalised the story, including a subplot about opium smuggling.

The book was not received particularly well.