Leoš Janáček (Czech: [ˈlɛoʃ ˈjanaːtʃɛk] , 3 July 1854 – 12 August 1928) was a Czech composer, music theorist, folklorist, publicist, and teacher. He was inspired by Moravian and other Slavic music, including Eastern European folk music, to create an original, modern musical style.

Born in Hukvaldy, Janáček demonstrated musical talent at an early age and was educated in Brno, Prague, Leipzig, and Vienna. He then returned to live in Brno, where he married his pupil Zdenka Schulzová and devoted himself mainly to folkloristic research. His earlier musical output was influenced by contemporaries such as Antonín Dvořák, but around the turn of the century he began to incorporate his earlier studies of national folk music, as well as his transcriptions of "speech melodies" of spoken language, to create a modern, highly original synthesis. The death of his daughter Olga in 1903 had a profound effect on his musical output; these notable transformations were first evident in the opera Jenůfa (often called the "Moravian national opera"), which premiered in 1904 in Brno.

In the following years, Janáček became frustrated with a lack of recognition from Prague, but this was finally relieved by the success of a revised edition of Jenůfa at the National Theatre in 1916, which gave Janáček access to the world's great opera stages. Janáček's later works are his most celebrated. They include operas such as Káťa Kabanová and The Cunning Little Vixen, the Sinfonietta, the Glagolitic Mass, the rhapsody Taras Bulba, two string quartets, and other chamber works. Many of Janáček's later works were influenced by Czech and Russian literature, his pan-Slavist sentiments, and his infatuation with Kamila Stösslová.

After his death in 1928, Janáček's work was heavily promoted on the world opera stage by the Australian conductor Charles Mackerras, who also restored some of his compositions to their original, unrevised forms. In his homeland he inspired a new generation of Czech composers including several of his students. Today he is considered one of the most important Czech composers, along with Dvořák and Bedřich Smetana.