The Secret of the Kingdom (original title Valtakunnan salaisuus) is a 1959 novel by Finnish author Mika Waltari about the early days of Christianity.

The story is told through the eyes of Marcus, a Roman citizen who arrives in Jerusalem on the day Jesus is crucified.

When he pays the customary courtesy call on the governor, Pontius Pilate, he learns that one of the three men, Jesus of Nazareth, is the leader of a religious sect suspected of sedition.

Kirkus Reviews described the novel as "a distinguished and original book", and wrote: "It is a moving story, with none of the shock techniques that one has come to expect of Waltari.

Payne continued: "I thought Lloyd Douglas had reached the extreme of vulgarity in The Robe, but Mika Waltari excels him.