Borno State is a state in the North-East geopolitical zone of Nigeria, bordered by Yobe to the west, Gombe to the southwest, and Adamawa to the south while its eastern border forms part of the national border with Cameroon, its northern border forms part of the national border with Niger, and its northeastern border forms all of the national border with Chad, being the only Nigerian state to border three foreign countries. It takes its name from the historic emirate of Borno, with the emirate's old capital of Maiduguri serving as the capital city of Borno State. The state was formed in 1976 when the former North-Eastern State was broken up. It originally included the area that is now Yobe State, which became a distinct state in 1991.
Borno is the second largest in area of the 36 states, only behind Niger State. Despite its size, the state is the eleventh most populous with an estimated population of about 5.86 million as of 2016. Geographically, the state is divided between the semi-desert Sahelian savanna in the north and the West Sudanian savanna in the centre and south with a part of the montane Mandara Plateau in the southeast. In the far northeast of the state is the Nigerian portion of Lake Chad and the Lake Chad flooded savanna ecoregion; the lake is fed by the Yobe River which forms the state's border with Niger until it reaches the lakebed. In the centre of the state is part of the Chad Basin National Park, a large national park that contains populations of black crowned crane, spotted hyena, patas monkey, and roan antelope along with transient herds of some of Nigeria's last remaining African bush elephants. However, a section of the park, the Sambisa Forest, was taken over during the Boko Haram insurgency in the early 2010s forcing many fauna to flee; large animals were not seen until 2019 and 2020 when a massive herd of migratory elephants returned to Borno.