Inspector French's Greatest Case is a 1924 mystery detective novel by Freeman Wills Crofts. It in his series of novels featuring Inspector French, a Scotland Yard detective of the Golden Age known for his methodical technique. Like much of the following series the plot mixes the traditional form of the puzzle mystery with that of a police procedural. French has to carefully study railway and shipping timetables and crack a cipher in order to solve his case.

Freeman Wills Crofts (1 June 1879 – 11 April 1957) was an Irish mystery author, best remembered for the character of Inspector Joseph French. A railway engineer by training, Crofts introduced railway themes into many of his stories, which were notable for their intricate planning. Although outshone by Agatha Christie, Raymond Chandler and other more celebrated authors from the golden age of detective fiction, he was highly esteemed by those authors, and many of his books are still in print.