The Three Impostors; or, The Transmutations is an episodic horror novel by Welsh writer Arthur Machen, first published in 1895 in The Bodley Head's Keynotes Series.

The novel comprises several weird tales and culminates in a denouement of deadly horror, connected with a secret society devoted to debauched pagan rites.

The three impostors of the title are members of this society who weave a web of deception in the streets of London, relating the aforementioned weird tales in the process.

They search for a missing Roman coin commemorating an infamous orgy by the Emperor Tiberius and close in on their prey: "the young man with spectacles".

In Things Near and Far (1923) Machen wrote: It was in the early spring of 1894 that I set about the writing of the said "Three Impostors," a book which testifies to the vast respect I entertained for the fantastic, "New Arabian Nights" manner of R. L. Stevenson, to those curious researches in the byways of London which I have described already, and also, I hope, to a certain originality of experiment in the tale of terror.Two of the stories in The Three Impostors, "The Novel of the Black Seal" and "The Novel of the White Powder", have often been anthologized as individual stories.