Kösem Sultan (Ottoman Turkish: كوسم سلطان; 1589 – 2 September 1651), also known as Mahpeyker Sultan (Ottoman Turkish: ماه پیكر;), was a Haseki Sultan (favorite) and legal wife of Ottoman sultan Ahmed I, Valide Sultan as mother of Murad IV and Ibrahim, and Büyük Valide Sultan as grandmother of Mehmed IV. She served as regent of the Ottoman Empire from 1623 to 1632 during the minority of Murad IV, from 1640 to 1648 during the unstable rule of Ibrahim, and again from 1648 until her assassination in 1651 during the minority of Mehmed IV. She became one of the most powerful and influential women in Ottoman history, as well as a central and controversial figure during the period known as the Sultanate of Women.

Kösem's status and influence were facilitated by her astute grasp of Ottoman politics and the large number of children she bore. Her sons and grandson required her regency, and her daughters' marriages to prominent statesmen provided her with allies in the government. She exerted considerable influence over Sultan Ahmed, and it's probable that her efforts on keeping his half-brother Mustafa — who later became Mustafa I — alive contributed to the transition from a system of succession based on primogeniture to one based on agnatic seniority.

Kösem was popular with the common citizens and esteemed by the ruling elite, holding significant political power and shaping the empire's domestic and foreign policies. Her early years as regent were marked by unrest and instability, but she nonetheless succeeded in maintaining the state institutions. In 1645, she pressured Sultan Ibrahim to launch a largely unsuccessful naval assault on the Venetian-controlled island of Crete. Afterwards, she had to contend with a Venetian blockade of the Dardanelles, which led to the naval Battle of Focchies in 1649, and in the years that followed, merchant upheavals brought on by a financial crisis.

Some historians openly attribute to Kösem a role in the demise of the Ottoman Empire. However, others argue that her policies can be interpreted as desperate attempts to avert a succession crisis and preserve the Ottoman dynasty. She was posthumously referred to by the names: Vālide-i Muazzama ("magnificent mother"), Vālide-i Maḳtūle ("murdered mother"), Umm al Mu'minīn ("mothers of believers"), and Vālide-i Şehīde ("martyred mother").

She was born around 1589. It is believed that she was Greek (from Tinos) or Bosnian. Apparently, the first report of Kösem's Greek origin was in a letter by the Italian traveler Pietro della Valle. On October 25, 1615, from Istanbul he wrote about Kösem Sultan: “She was the daughter, if I understood correctly, of a Greek priest, from a distant country or city about two hundred miles from Constantinople”. Her Christian name is unknown, but a popular version (as stated in unauthorized sources) is that her name was Anastasia.

In 1604, at the age of 14 or 15, she was kidnapped by Ottoman raiders and bought as a slave in Bosnia by the beylerbey (governor-general) of the Bosnia Eyalet. She was a tall, slender, and appealing girl due to her fresh complexion and the deep brown of her eyes. Her beauty and intelligence were noticed by the kızlar ağa of Sultan Ahmed I's court, who sent her to Constantinople to join a group of other slave girls marked by their striking appearance or intelligence to be trained in the harem of Sultan Ahmed I as an imperial court lady (slave concubine).

In the harem, she was taught Islam, theology, mathematics, embroidery, singing, music and literature. Ahmed was captivated by her beauty and intelligence, and in 1605, she became his haseki.

The pair apparently married at some point, as a letter to the Venetians written during Murad IV's reign states that Ahmed had wed Kösem; nevertheless surviving contemporary evidence surrounding the exact circumstances of their marriage is scarce.