"The Blue Cross" is a short story by G. K. Chesterton. It was the first Father Brown short story and also introduces the characters Hercule Flambeau and Aristide Valentin. It is unique among the Father Brown mysteries in that it does not follow the actions of the Father himself, but rather those of Valentin. It was first published on 23 July 1910, under the title "Valentin Follows a Curious Trail", in the Saturday Evening Post, Philadelphia. Re-titled as "The Blue Cross", publication in London followed, in The Story-Teller magazine of September 1910.

Father Brown is a fictional Roman Catholic priest and amateur detective. He is featured in 53 short stories by English author G. K. Chesterton, published between 1910 and 1936. Father Brown solves mysteries and crimes using his intuition and keen understanding of human nature. Chesterton loosely based him on the Rt Rev. Msgr John O'Connor (1870–1952), a parish priest in Bradford, who was involved in Chesterton's conversion to Catholicism in 1922. Since 2013, the character has been portrayed by Mark Williams in the ongoing BBC Television Series Father Brown.