A Janissary (Ottoman Turkish: یڭیچری, romanized: yeŋiçeri, [jeniˈtʃeɾi], lit. 'new soldier') was a member of the elite infantry units that formed the Ottoman Sultan's household troops and the first modern standing army in Europe. The corps was most likely established under sultan Orhan (1324–1362), during the Viziership of Alaeddin.

Janissaries began as elite corps made up through the devşirme system of child levy enslavement, by which Christian Albanians, Bulgarians, Croats, Greeks, Romanians and Serbs were taken, levied, subjected to forced circumcision and conversion to Islam, and incorporated into the Ottoman army. They became famed for internal cohesion cemented by strict discipline and order. Unlike typical slaves, they were paid regular salaries. Forbidden to marry before the age of 40 or engage in trade, their complete loyalty to the Sultan was expected. By the seventeenth century, due to a dramatic increase in the size of the Ottoman standing army, the corps' initially strict recruitment policy was relaxed. Civilians bought their way into it in order to benefit from the improved socioeconomic status it conferred upon them. Consequently, the corps gradually lost its military character, undergoing a process that has been described as "civilianization".

The Janissaries were a formidable military unit in the early centuries, but as Western Europe modernized its military organization and technology, the Janissaries became a reactionary force that resisted all change. Steadily the Ottoman military power became outdated, but when the Janissaries felt their privileges were being threatened, or outsiders wanted to modernize them, or they might be superseded by their cavalry rivals, they rose in rebellion. By the time the Janissaries were suppressed, it was too late for Ottoman military power to catch up with the West. The corps was abolished by Sultan Mahmud II in 1826 in the Auspicious Incident, in which 6,000 or more were executed.