The Makropulos Affair (or The Makropoulos Case, or The Makropulos Secret or, literally, The Makropulos Thing; Czech Věc Makropulos) is a three-act opera by Czech composer Leoš Janáček. The libretto, based on a play of the same name by Karel Čapek, was written by the composer between 1923 and 1925.

The Makropulos Affair was his penultimate opera and, like much of his later work, it was inspired by his infatuation with Kamila Stösslová, a married woman much younger than himself.

The world premiere of the opera was given at the National Theatre in Brno on December 18, 1926, conducted by František Neumann.

Janáček's operatic version was written between 1923 and 1925. He had seen the play early in its run in Prague on 10 December 1922, and immediately saw its potential. He entered into a correspondence with Čapek, who was accommodating towards the idea, although legal problems in securing the rights to the play delayed work. When these problems were cleared on 10 September 1923, Janáček began work on the opera straight away. He wrote the libretto himself, and by December 1924 had completed the first draft of the work. He spent another year refining the score, before completing it on 3 December 1925.

Musically, much of the piece has little in the way of thematic development, instead presenting the listener with a mass of different motifs and ideas. Janáček's writings indicate that this was a deliberate ploy to give musical embodiment to the disruptive, unsettling main character Emilia Marty/Elina Makropulos. Only at the end of the final act, when Makropulos' vulnerability is revealed, does the music tap into and develop the rich lyrical vein that has driven it throughout.

It is often argued that Emilia Marty, like the other female heroes in Janáček's later operas, stands for one of the aspects of Kamila Stösslová, the woman with whom he was in love for the last decade of his life. Marty, with a clever and manipulative exterior hiding a core of vulnerability, is a 'snapshot' of Stösslová, like the coquettish and shy Cunning Little Vixen and the tragic Káťa Kabanová.