Janata Dal (United) ("People's Party (United)"), abbreviated as JD(U), is a social-democratic and secular Indian political party, rooted mainly in eastern and north-eastern India, whose stated goals are promoting social justice and lifting up marginalised people. JD(U) is recognised as a state party in the states of Bihar, where it heads the government, Manipur, where it is part of the government coalition in the legislative assembly, and Arunachal Pradesh. JD(U), as part of the Bharatiya Janata Party-led National Democratic Alliance, won 12 seats in the 2024 Indian general election, making it the seventh largest party in the Lok Sabha.

JD(U) was formed with the merger of Sharad Yadav's faction of the Janata Dal, the Samata Party and the Lok Shakti party on 30 October 2003. The party's mentor and patron was the veteran socialist George Fernandes, successively leader of the Samyukta Socialist Party, the Socialist Party and the Samata Party. However, the Election Commission of India initially refused the merger of the latter: party president Brahmanand Mandal was suffering from Alzheimer's disease and not physically well, so Uday Mandal replaced him.

The Janata Dal (United)'s origin goes back to the period before 1999 General Election. A faction led by then Chief Minister of Karnataka J. H. Patel had lent support to the National Democratic Alliance, leading to the split in the Janata Dal and the formation of Janata Dal (Secular) under H. D. Deve Gowda, who wanted to remain equidistant from both national parties; and Janata Dal under Sharad Yadav was called Janata Dal (United).

The Janata Dal (United) was formed with the merger of the Sharad Yadav faction of the Janata Dal, the Lok Shakti and the Samata Party. On 30 October 2003, the Samata Party led by George Fernandes and Nitish Kumar merged with the Janata Dal. The merged entity was called Janata Dal (United) with the arrow symbol of Janata Dal (United) and the green and white flag of the Samata Party. The uniting force is believed to be common opposition to Rashtriya Janata Dal in Bihar especially after the Rashtriya Janata Dal welcomed Samata Party rebels like Raghunath Jha into the party.

JD(U) joined NDA and along with its alliance partner, the BJP defeated the RJD-led UPA government in Bihar in November 2005. The new government was headed by JD(U) leader, Nitish Kumar and NDA continued to govern state. The alliance contested 2009 Indian general election and won 32 seats. BJP won 12 while JD(U) won 20. JD(U) won 115 and BJP won 91 seats in 2010 Bihar Legislative Assembly election. Thus together holding 206 seats in 243 member Bihar Legislative Assembly.

JD(U) broke its 17 years old alliance with the BJP in Bihar in protest against the elevation of Narendra Modi as ahead of the election campaign committee of BJP for 2014 Indian general election. JD(U) President Sharad Yadav and then Bihar Chief Minister Nitish Kumar announced their end of coalition at a press conference on 16 June 2013, exactly a week after Narendra Modi was made the BJP's campaign committee chairman, who was later made the prime ministerial candidate of NDA. Just after this split, Sharad Yadav relinquished his position as the NDA convenor.

The JD(U) contested the election in Bihar in an alliance with the Communist Party of India but they won only two seats out of total forty seats of Bihar while the BJP-LJP alliance won 31 seats. Following poor performance in election, Nitish Kumar resigned as Chief Minister of Bihar and Jitan Ram Manjhi sworn in as a new Chief Minister. When the trust vote was demanded by the BJP to prove majority in Bihar Legislative Assembly, the RJD supported the JD(U) in the assembly on 23 May 2014 to pass the majority mark.

On 29 December 2014, Kerala-based Socialist Janata (Democratic) merged with the JD(U) with its leader M.P. Veerendra Kumar, accepting the party flag from JD(U) leader Nitish Kumar. This was an important milestone in bringing a pan-Indian appeal to the JD(U) which is largely limited to the state of Bihar.