The history of Sierra Leone began when the land became inhabited by indigenous African peoples at least 2,500 years ago. The dense tropical rainforest partially isolated the region from other West African cultures, and it became a refuge for peoples escaping violence and jihads. Sierra Leone was named by Portuguese explorer Pedro de Sintra, who mapped the region in 1462. The Freetown estuary provided a good natural harbour for ships to shelter and replenish drinking water, and gained more international attention as coastal and trans-Atlantic trade supplanted trans-Saharan trade.

In the mid-16th century, the Mane people invaded, subjugated nearly all of the indigenous coastal peoples, and militarised Sierra Leone. The Mane soon blended with the local populations and the various chiefdoms and kingdoms remained in a continual state of conflict, with many captives sold to European slave-traders. The Atlantic slave trade had a significant impact on Sierra Leone, as this trade flourished in the 17th and 18th centuries, and later as a centre of anti-slavery efforts when the trade was abolished in 1807. British abolitionists had organised a colony for Black Loyalists at Freetown, and this became the capital of British West Africa. A naval squadron was based there to intercept slave ships, and the colony quickly grew as liberated Africans were released, joined by West Indian and African soldiers who had fought for Britain in the Napoleonic Wars. The descendants of the black settlers were collectively referred to as the Creoles or Krios.

During the colonial era, the British and Creoles increased their control over the surrounding area, securing peace so that commerce would not be interrupted, suppressing slave-trading and inter-chiefdom war. In 1895, Britain drew borders for Sierra Leone which they declared to be their protectorate, leading to armed resistance and the Hut Tax War of 1898. Thereafter, there was dissent and reforms as the Creoles sought political rights, trade unions formed against colonial employers, and peasants sought greater justice from their chiefs.

Sierra Leone has played a significant part in modern African political liberty and nationalism. In the 1950s, a new constitution united the Crown Colony and Protectorate, which had previously been governed separately. Sierra Leone gained independence from the United Kingdom in 1961 and became a member of the Commonwealth of Nations. Ethnic and linguistic divisions remain an obstacle to national unity, with the Mende, Temne and Creoles as rival power blocs. Roughly half of the years since independence have been marked by autocratic governments or civil war.