Portuguese traders likewise ousted Arab merchants as the prime purveyors of African slaves. Coerced labor — through slavery, serfdom, or indentured servitude — was the norm in most premodern societies, and in Africa slavery was widespread. Some Africans were held in bondage as security for debts; others were sold into servitude by their kin in exchange for food in times of famine; many others were war captives. Slaves were a key commodity of exchange, sold as agricultural laborers, concubines, or military recruits. Sometimes their descendants were freed, but others endured hereditary bondage. Sonni Ali (r. 1464–1492), the ruler of the powerful Songhai Empire, personally owned twelve “tribes” of hereditary agricultural slaves, many of them seized in raids against stateless peoples.

Slaves were also central to the trans-Saharan trade. When the renowned Tunisian adventurer Ibn Battuta crossed the Sahara from the Kingdom of Mali around 1350, he traveled with a caravan of six hundred female slaves, destined for domestic service or concubinage in North Africa, Egypt, and the Ottoman Empire. Between A.D. 700 and 1900, it is estimated that as many as nine million Africans were sold in the trans-Saharan slave trade.

Europeans initially were much more interested in trading for gold and other commodities than in trading for human beings, but gradually they discovered the enormous value of human trafficking. To exploit and redirect the existing African slave trade, Portuguese merchants established fortified trading posts like those in the Indian Ocean beginning at Elmina in 1482, where they bought gold and slaves from African princes and warlords. First they enslaved a few thousand Africans each year to work on sugar plantations on São Tomé, Cape Verde, the Azores, and Madeira; they also sold slaves in Lisbon, which soon had an African population of 9,000. After 1550, the Atlantic slave trade, a forced diaspora of African peoples, expanded enormously as Europeans set up sugar plantations in Brazil and the West Indies.