The Lurker at the Threshold is a Cthulhu Mythos novel, credited on its publication by Arkham House in 1945 to H. P. Lovecraft and August Derleth, although almost all of it was written by Derleth, incorporating "notes and fragments" written by Lovecraft.

The Lurker at the Threshold is divided into three sections. "Billington's Wood" focuses on Ambrose Dewart, an English man who reclaims an ancestral property near Arkham, Massachusetts, and learns the disquieting history of his ancestors Richard and Alijah Billington. Disregarding the latter's posthumous warning, he removes a stone from a round tower on the Billington property, and thus unleashes eldritch forces.

The second section, "Manuscript by Stephen Bates," is presented as an account by Dewart's cousin, who comes to realize that Dewart is under a malignant influence.

The conclusion, "Narrative of Winfield Phillips," describes the actions of Miskatonic University librarian Seneca Lapham to deal with the evil unleashed by Dewart--written from the perspective of Lapham's assistant.