Cassava was introduced at a number of points along the West African coast during the 17th century, from the Gambia River to present-day Nigeria. Portuguese forts, trading posts and settlements were founded on the mainland and, by the end of the 17th century, cassava was present at most of these places.

Unlike Central Africa, the diffusion of cassava in West Africa was universally slow, and most of the crop's spread took place during the late 19th and 20th centuries. The principal reason was the human geography and political organization of the West African kingdoms which differed markedly from those of Central Africa. The humid coastal belt was essentially uninhabited, and formed a peripheral zone about inland capitals.

Whilst cassava may have spread inland along the Gambia River, it did not penetrate northwards along the Niger from the Portuguese station at Warri until very late. Jones notes that innovations tended to spread from the northern capitals to their southern peripheries rather than vice versa, and most West African peoples had no crops similar to cassava nor knowledge of the necessary processing techniques.

Notwithstanding this, we find occasional references to the adoption of cassava in various parts of West Africa prior to the 19th century. Although cassava seems to have been absent along the Gold Coast (Ghana) at the beginning of the 18th century, it was widely cultivated around Accra in 1785. We have insufficient information about the Guineas, Liberia and Sierra Leone to piece together the crop's diffusion in the westernmost part of the continent.

More information is available about the spread of cassava in Nigeria and Benin. The growth of cassava production and its diffusion in these countries are attributed to the catalytic effect of freed Brazilian slaves who began to return to the area around 1800. In some areas, however, cassava was already cultivated before returned slaves had visited the area. Agboola (1968) hypothesized the introduction of cassava in Benin and Nigeria in either the 17th or 18th century.

At Ouidah, in present-day Benin, the Portuguese maintained a factory staffed by Brazilians. This was the most likely source of cassava introduced to south-west Nigeria, probably with the original intention of supplying slave ships with farinha.

Igbo migration was an important diffusion mechanism in the eastern states of Nigeria. The Igbo must have been in contact with cassava since the 17th century after its introduction at Owerri.

The spread of cassava from coastal to inland areas remains obscure. Cassava was at first used as a medicine in Benin, as a cure for tuberculosis. Pasch (1980) found that from Ivory Coast (Côte d'lvoire) to the Niger River, the names for cassava resembled each other closely, pointing to a common origin. She attributed great importance to the Mande peoples' diffusion of cassava in West Africa.

The Bambara were probably very influential in cassava diffusion under the Mande. One hypothesis is that the Mande took cassava from the coast to the east. In northerly areas of West Africa (among the Hausa, for example), Arabic names for cassava are more common, suggesting a route through these tribes.

Slaves who returned from Brazil from the late 18th century onwards were certainly also instrumental in the spread of cassava. They became an urban class, at first controlling the slave trade, who created a local demand for cassava. Jones thought that, at the same time, they introduced the necessary processing techniques to detoxify bitter roots, although further investigation is required to determine whether processing techniques were totally unknown before the slaves' arrival.

The cassava product 'gari' was most likely introduced by slaves. At first, Africans would not have been able to distinguish the bitter from sweet varieties of the new crop. Without the means to process bitter roots, it is easy to understand their unwillingness to adopt a potentially lethal new crop.

The consumption habits and preferences of Brazilians and freed slaves and their knowledge of cultivation and processing, led to the spread of cassava production through a neighborhood effect. Urban lifestyles and the growth of a working class in the Lagos area increased demand, and local peoples emulated the habits of the Afro-Brazilians.

By mid-19th century, Badagry, Abeokuta, Lagos and Ijebu were centers of production. By 1860, the crop had reached Ibadan, and the area of production slowly coalesced. Nevertheless, expansion was slow, because of fear of poisoning, and cassava remained a cash crop for the urban markets of Lagos in most areas where it was grown.

Early travel accounts have shown that cassava was known in northern Nigeria in 1850. In 1825, it had not been recorded by early European travelers, suggesting the entrance of cassava in about 1830-1840. It may well be that cassava reached northern Nigeria via Central Africa, through the migrations of the Fulani, rather than from coastal West Africa.

In any case, cassava was unimportant north of the Niger-Benue confluence until after World War I, and was still only of limited importance in Oyo State in 1951-1952.

As in many countries outside Central Africa, in Nigeria, cassava spread most rapidly during the 20th century. To a large extent, this was a result of governmental encouragement, because of the crop's resistance to locust attacks and drought and its consequent value as a famine reserve.

In Nigeria, the construction of north-to-south railway arteries and labor migration to the coast accelerated diffusion and increased the inland peoples' exposure to the crop. The easy incorporation of cassava into Nigerian farming systems can be attributed to its low labor requirements during growth and the flexibility of its harvest period.

Although Jones thought soil degradation to be a reason favoring cassava's adoption, Agboola maintains that there are no sources pointing to depletion of soil resources as a historical reason for cassava's diffusion in Nigeria. Where cassava's increasing importance was associated with declining fallow lengths, the latter were a result of the expansion of tree crops from the 1920s onwards.

The way in which cassava replaced and became more important than yam confirms that labor constraints and, to a lesser extent, market demand have been more important contemporary factors in the diffusion of cassava in Nigeria.

In West Africa as a whole, colonial governments played a major role in encouraging cassava cultivation during the 20th century, particularly in the savanna areas.