The King of The Copper Mountains is a children's novel by Paul Biegel, and was originally published as Het Sleutelkruid in the Netherlands in 1964, where it won the Gouden Griffel, the award for the best children's book of the year.

An English translation by the author with illustrations by Gilliam Hume was published in 1968 by J. M. Dent.

The book received widespread acclaim in Europe, won the Dutch literary prize for children's literature, and is now considered a classic.

Amanda Craig described it in The Times in 2008 as "one of the blazing jewels of children's literature.

"[1] A paperback re-issue by Glasgow-based Strident Publishing from 2008 includes new illustrations and an introduction by the author's daughter Leonie.