The Kizlar Agha or Aga (Ottoman Turkish: قيزلر اغاسی, Turkish: Kızlar Ağası, "Agha of the [slave] Girls"), formally the Agha of the House of Felicity (Arabic: Aghat Dar al-Sa'ada, Turkish: Darüssaade ağa), was the head of the eunuchs who guarded the Imperial Harem of the Ottoman Sultans in Constantinople. Due to his proximity to the Sultan and the role the harem ladies played in court intrigues, the post ranked among the most important in the Ottoman Empire until the early 19th century. Soon after its creation and until its abolition at the end of the Ottoman Empire, the post came to be occupied by Black African eunuch slaves, and hence is also referred to as the Chief Black Eunuch.
The post of the Kizlar Agha was created in the reign of Murad III (ruled 1574–1595) in 1574, with the Ethiopian Mehmed Agha as its first occupant. Until then, the Ottoman palace had been dominated by the white eunuchs, chiefly drawn from the Christian populations of the Balkans or the Caucasus. The 16th century, however, saw a rapid rise of the population of the Topkapi Palace, including among eunuchs, whose numbers rose from 40 under Selim (r. 1512–1520) to over a thousand under Murad III. While black eunuchs had served alongside white eunuchs in the palace, by 1592, for reasons that are unclear, both a separation of roles as well as the ascendancy of the black eunuchs over the white ones had become established: white eunuchs were restricted to the supervision of the male pages (içoğlan), while black eunuchs took over the far more prestigious supervision of the private apartments of the Sultan and the palace women (harem). Consequently, the Chief Black Eunuch quickly eclipsed the "Chief White Eunuch" or Kapi Agha (kapı ağası, "Agha of the Gate"), who had hitherto been the head of the palace personnel, and rose to become, in the words of the Orientalist C. E. Bosworth, "in practice the principal officer of the whole palace". At the height of the post's power in the 17th and 18th centuries, the Kizlar Agha was a vizier of the first rank ("with three horsetails") and came third in the state hierarchy, next only to the Empire's chief minister, the Grand Vizier, and the chief religious authority, the Sheikh ul-Islam.