The Blue Castle is a 1926 novel by Canadian author Lucy Maud Montgomery, best known for her novel Anne of Green Gables (1908).
The story is set during the early 1900s in the fictional town of Deerwood, located in the Muskoka region of Ontario, Canada. Deerwood is based on Bala, Ontario, which Montgomery visited in 1922. Maps of the two towns show similarities.

This novel is considered one of L.M. Montgomery's few adult works of fiction, along with A Tangled Web, and is the only book she wrote that is entirely set outside of Prince Edward Island. It has grown in popularity since being republished in 1990. The book was adapted for the stage twice; in 1982 it was made into a successful Polish musical, and ten years later Canadian playwright Hank Stinson authored another version, The Blue Castle: A Musical Love Story.

Valancy is, at twenty-nine, the old maid of the Stirling clan, which is a reputable family that has lived in the same region for over fifty years. As an only child, her entire life has been spent with her nagging mother, her perpetually down-trodden aunt, and a gossipy extended family, who, in spirit of the Victorian and middle class, actively discourage happiness and treat Valancy like a child, telling her what to do every day and constantly comparing her with her beautiful cousin, Olive, putting her down, and calling her by the baby name "Doss." Her only respites come from daydreaming of her "Blue Castle" and the handsome men who would reside in it with her, along with reading the nature books of John Foster.