The Mysterious Affair at Styles is a detective novel by British writer Agatha Christie. It was her first published novel, written in 1916, and published in the US and in the UK a few years later. It introduced Hercule Poirot, Inspector (later, Chief Inspector) Japp, and Arthur Hastings.

This first mystery novel by Agatha Christie was well received by reviewers. An analysis in 1990 was positive about the plot, considered the novel one of the few by Christie that is well-anchored in time and place, a story that knows it describes the end of an era, and mentions that the plot is clever. Christie had not mastered cleverness in her first novel, as "too many clues tend to cancel each other out"; this was judged a difficulty "which Conan Doyle never satisfactorily overcame, but which Christie would."