The Funj Sultanate, also known as Funjistan, Sultanate of Sennar or Blue Sultanate was a kingdom in what is now Sudan, northwestern Eritrea and western Ethiopia. Founded in 1504 by the Funj people, it quickly converted to Islam, although this conversion was only nominal. Until a more orthodox form of Islam took hold in the 18th century, the state remained an "African empire with a Muslim façade", coming to rule over an ethnically diverse population.

At its greatest extent the polity extended from the Third Cataract southwards to the Ethiopian Highlands and Sobat River, east to the Red Sea, and west to Kordofan and the Nuba Mountains. It reached its peak in the late 17th century, but declined and eventually fell apart in the 18th and 19th centuries. In 1821, the last sultan, greatly reduced in power, surrendered to the Ottoman Egyptian invasion without resistance.

Christian Nubia, represented by the two medieval kingdoms of Makuria and Alodia, began to decline from the 12th century. By 1365 Makuria had virtually collapsed and was reduced to a rump state restricted to Lower Nubia, until finally disappearing c. 150 years later. The fate of Alodia is less clear. It has been suggested that it had collapsed as early as the 12th century or shortly after, as archaeology suggests that in this period, Soba ceased to be used as its capital. By the 13th century central Sudan seemed to have disintegrated into various petty states. Between the 14th and 15th centuries Sudan was overrun by Bedouin tribes who introduced Islam and the Arabic language to the region. In the 15th century one of these Bedouins, whom Sudanese traditions refer to as Abdallah Jammah (of the Abdallabi tribe), is recorded to have created a tribal federation and to have subsequently destroyed what was left of Alodia. In the early 16th century Abdallah's federation came under attack from an invader to the south, the nomadic-pastoralist Funj, who followed their own traditional religion.

The origins and ethnic affiliation of the Funj are still disputed, and scholars have expressed doubt that it will ever be solved. Some scholars theorise that they were either Nubians or Shilluk, while others contend that the Funj were not an ethnic group, but a social class. Other possible origins include in Bornu, Eritrea, or northern Ethiopia. Sudanese tradition varies, with some claiming the Funj rulers descended from the Umayyads.

In the 14th century a Muslim Funj trader named al-Hajj Faraj al-Funi was involved in the Red Sea trade. According to oral traditions the Dinka, who migrated upstream the White and Blue Nile since the 13th-century disintegration of Alodia, came into conflict with the Funj, who the Dinka defeated. In the late 15th/early 16th century the Shilluk arrived at the junction of the Sobat and the White Nile, where they encountered a sedentary people Shilluk traditions refer to as Apfuny, Obwongo and/or Dongo, a people now equated with the Funj. Said to be more sophisticated than the Shilluk, they were defeated in a series of brutal wars and either assimilated or pushed north.

There are two accounts on the founding of the Funj Sultanate in 1504. The first of which appears in the Funj Chronicle (dated to the 19th century) wherein Funj chief Amara Dunqas is said to have allied Abdallah Jammah to conquer Soba, however scholars consider the Abdallabi to have conquered Soba earlier on their own. The second, as reported by James Bruce, details a battle near Arbadji between the two parties. Regardless, the Funj Sultanate was founded with the Abdallabi chiefs assuming subordinate positions, and the Funj quickly Islamised.

In 1523 the kingdom was visited by Jewish traveller David Reubeni, who disguised himself as a Sharif. Ruler Amara Dunqas, Reubeni wrote, was Muslim and continuously travelled through his kingdom. He, who "ruled over black people and white" between the region south of the Nile confluence to as far north as Dongola, owned large herds of various types of animals and commanded many captains on horseback.

In 1525, Ottoman admiral Selman Reis mentioned Amara Dunqas and his kingdom, calling it weak and easily conquerable. He also stated that Amara paid an annual tribute of 9,000 camels to the Ethiopian Empire. One year later the Ottomans occupied Sawakin, which beforehand was associated with Sennar. It seems that to counter the Ottoman expansion in the Red Sea region, the Funj engaged in an alliance with Ethiopia. Besides camels the Funj are known to have exported horses to Ethiopia, which were then used in war against the Adal Sultanate. The borders of Funj were raided by Ahmed Gurey during the war taking many slaves before stopping near the Taka mountain range near modern-day Kassala.