Bihar (; Hindi pronunciation: [bɪˈɦaːr] (listen)) is a state in eastern India. It is the third largest state by population, the 12th largest by area, and the 14th largest by GDP in 2021. Bihar borders Uttar Pradesh to its west, Nepal to the north, the northern part of West Bengal to the east, and with Jharkhand to the south. The Bihar plain is split by the river Ganges, which flows from west to east.

On 15 November 2000, southern Bihar was ceded to form the new state of Jharkhand. Only 20% of the population of Bihar lives in urban areas as of 2021. Additionally, almost 58% of Biharis are below the age of 25, giving Bihar the highest proportion of young people of any Indian state. The official language is Hindi and additional language is Urdu, although other languages are common, including Maithili, Magahi, Bhojpuri, and other Languages of Bihar.

In Ancient and Classical India, the area that is now Bihar was considered the centre of political and cultural power and as a haven of learning. From Magadha arose India's first empire, the Maurya empire, as well as one of the world's most widely adhered-to religions: Buddhism. Magadha empires, notably under the Maurya and Gupta dynasties, unified large parts of South Asia under a central rule. Another region of Bihar, Mithila, was an early centre of learning and the centre of the Videha kingdom.

However, since the late 1970s, Bihar has lagged far behind other Indian states in terms of social and economic development. Many economists and social scientists claim that this is a direct result of the policies of the central government, such as the freight equalisation policy, its apathy towards Bihar, lack of Bihari sub-nationalism, and the Permanent Settlement of 1793 by the British East India Company. The state government has, however, made significant strides in developing the state. Improved governance has led to an economic revival in the state through increased investment in infrastructure, better healthcare facilities, greater emphasis on education, and a reduction in crime and corruption.