Fonio, also sometimes called findi or acha, is the term for two cultivated grasses in the genus Digitaria that are important crops in parts of West Africa. It is a vital food source in many rural areas, especially in the mountains of Fouta Djalon, Guinea, but it is also cultivated in Mali, Burkina Faso, Ivory Coast, Nigeria, and Senegal. The global fonio market was estimated at 721,400 tonnes in 2020. Guinea annually produces the most fonio in the world, accounting for over 75% of the world's production in 2019. The name fonio (borrowed into English from French) is from Wolof foño. In West Africa, the species black fonio (Digitaria iburua) and white fonio (Digitaria exilis) are cultivated; the latter is the economically more important crop.

Fonio is a glumaceous monocot belonging to the grass family Poaceae and the genus Digitaria. While hundreds of these crabgrass species exist, only a few of them are produced for their grains. It is a small annual herbaceous plant with an inflorescence containing two or three racemes. The racemes have spikelets grouped in twos, threes, or fours, with a sterile and a fertile flower producing the fonio grain. Fonio has a short growing season and is well adjusted to harsh environments. The size of its root system, which can extend down to more than one meter in depth, is advantageous in periods of drought and helps with its adaptation to poor soils. Once considered a humble and often overlooked grain commonly known as the "cereal of the poor," fonio is now gaining attention in urban West Africa. Its unique cooking properties and nutritional benefits are sparking renewed interest in this once underrated staple.

White fonio, Digitaria exilis, also called "hungry rice" by Europeans, is the most common of a diverse group of wild and domesticated Digitaria species that are harvested in the savannas of West Africa. Fonio has the smallest seeds of all species of millet. It has potential to improve nutrition, boost food security, foster rural development, and support sustainable use of the land.

Nutritious, gluten-free, and high in dietary fiber, fonio is one of the world's fastest-growing cereals, reaching maturity in as little as six to eight weeks. The grains are used to make porridge, couscous, bread, and beer.

Black fonio, D. iburua, also known as iburu, is a similar crop grown in several countries of West Africa, particularly Nigeria, Togo, and Benin. Like white fonio, it is nutritious, fast-growing, and has the benefit of maturing before other grains, allowing for harvest during the "hungry season." However, it contains considerably more protein compared to D. exilis.

Black fonio is mostly cultivated in rural communities and is rarely sold commercially, even in West African cities.

Fonio is cultivated in all West Africa as a staple crop. Guinea is the biggest producer of fonio with a production of 483,906 tonnes (533,415 short tons) and a cultivated surface area of 590,129 hectares (1,458,240 acres) in 2021, followed by Nigeria (86,609 tonnes (95,470 short tons)) and Mali (47,664 tonnes (52,541 short tons)).

Fonio grows in dry climates without irrigation, and is unlikely to be a successful crop in humid regions. It is planted in light (sandy to stony) soils, and will grow in poor soil. Fonio is cultivated at sea level in Gambia, Sierra Leone and Guinea-Bissau, but it is otherwise mostly cultivated in altitudes ranging between 400 and 1,500 metres (1,300 and 4,900 ft). The growth cycle ranges from 70–130 days, depending on variety. It is mostly grown in areas with an average annual rainfall of 900–1,000 millimetres (35–39 in).