A janissary (UK: /ˈdʒænɪsəri/ JAN-iss-ər-ee, US: /-sɛri/ -⁠err-ee; Ottoman Turkish: یڭیچری, romanized: yeñiçeri, IPA: [jeˈŋit͡ʃeɾ̞i], lit. 'new soldier') was a member of the elite infantry units that formed the Ottoman sultan's household troops. They were the first modern standing army, and perhaps the first infantry force in the world to be equipped with firearms, adopted during the reign of Murad II (r. 1421–1444, 1446–1451). The corps was established under either Orhan or Murad I, and dismantled by Mahmud II in 1826.

Janissaries began as an elite corps made up through the devşirme system of child levy enslavement, by which Christian boys, chiefly from the Balkans, were taken, levied, subjected to forced circumcision and forced 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 Ottoman sultan was expected. By the 17th 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 socio-economic status it conferred upon them. Consequently, the corps gradually lost its military character, undergoing a process that has been described as "civilianization".

The Janissary Corps 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 within the Ottoman army. Steadily the Ottoman military power became outdated, but when the Janissaries felt their privileges were being threatened, or outsiders wanted to modernize them, or they might be superseded by their cavalry rivals, they would rise in rebellion. By the time the Janissaries were suppressed, it was too late for Ottoman military power to catch up with the West. The Janissary Corps was abolished by Mahmud II in 1826 in the Auspicious Incident, in which 6,000 or more Janissaries were executed.

The Janissary Corps was formed in the 14th century, either during the rule of Murad I (r. 1362–1389), the third sultan of the Ottoman Empire, or during the time of Murad's father, Orhan (r. c. 1324 – 1362). The Ottoman government instituted a tax of one-fifth on all slaves taken in war, and from this pool of manpower the sultans first constructed the Janissary corps as a personal army loyal only to the Ottoman sultan.

From the 1380s to 1648, the Janissaries were gathered through the devşirme system of child levy enslavement, which was abolished in 1648. This recruitment of Janissary troops was achieved through the enslaving of dhimmi peoples (i.e., non-Muslims), predominantly Balkan Christians. Jews were never subject to devşirme; however, there is evidence that Jews tried to enroll into the system. Jews were not allowed to join the Janissary Corps, and so in suspected cases the entire batch would be sent to the Imperial Arsenal as indentured laborers. Ottoman documents from the levy of the winter of 1603-1604 from Bosnia and Albania wrote to draw attention to some children as "possibly being Jewish" (şekine-i arz-ı yahudi). According to the Encyclopedia Britannica, "in early days, all Christians were enrolled indiscriminately. Later, those from what is now Albania, Bosnia, Serbia, Greece, Bulgaria and Hungary were preferred." The Bektashi Order became the official religious and spiritual institution of the Janissaries in the 15th century.

The Janissaries were kapıkulları (sing. kapıkulu), "door servants" or "slaves of the Porte", neither freedmen nor ordinary slaves (köle). They were subjected to strict discipline, but were paid salaries and pensions upon retirement and formed their own distinctive social class. As such, they became one of the ruling classes of the Ottoman Empire, rivalling the Ottoman Turkish aristocracy. The brightest of the Janissaries were sent to the palace institution, Enderun. Through a system of meritocracy, the Janissaries held enormous power, stopping all efforts to reform the military.

According to military historian Michael Antonucci and economic historians Glenn Hubbard and Tim Kane, the Turkish administrators would scour their provinces (but especially the Balkans) every five years for the strongest sons of the European Christians. When a non-Muslim boy was recruited under the devşirme system of child levy enslavement, he would first be sent to selected Ottoman Turkish families in the provinces to learn Turkish, subjected to forced circumcision and forced conversion to Islam, and to learn the customs and culture of Ottoman society. After completing this period, acemi ("new recruit") boys were gathered for training at the Enderun acemi oğlan ("rookie" or "cadet") school in the capital city. There, young cadets would be selected for their talents in different areas to train as engineers, artisans, riflemen, clerics, archers, artillery, and so forth. Most were of non-Muslim origin because it was not permissible to enslave other Muslims.

It was a similar system to the Iranian Safavid, Afsharid, and Qajar-era ghilman, who were drawn from converted Circassians, Georgians, and Armenians, and in the same way as with the Ottoman Janissaries, who had to replace the unreliable ghazi. They were initially created as a counterbalance to the tribal, ethnic, and favoured interests the Qizilbash gave, which make a system imbalanced.