Samajwadi Party (literally, Socialist Party) is a political party in India. It is based in the Indian state of Uttar Pradesh. It describes itself as a democratic socialist party. It was founded on October 4, 1992. It is one of several parties that emerged when the erstwhile Janata Dal (People's League), India's primary opposition party prior to the BJP, was fragmented into several regional parties. Samajwadi Party is led by Mulayam Singh Yadav, a former Chief Minister of Uttar Pradesh and a former defence minister of the country.

Samajwadi Party is primarily based in Uttar Pradesh, where it bases its support largely on OBCs (Other Backward Castes) and Muslims, particularly Mulayam Singh Yadav's own Yadav caste. It has made strong attempts to gain national status, by contesting Lok Sabha and state Assembly elections around the country, but it has been unsuccessful thus far. In last assembly election in Madhya Pradesh in 2003, Samajwadi Party did well and gained 7 seats which took it as a third largest party in the state. By winning the bye-election of Lanji (Balaghat) in 2007, its total tally reached 8.

In the 14th Lok Sabha, it currently has 38 members, and is the fourth largest party in that house, its best ever tally. Besides 36 members from Uttar Pradesh, it won one seat from Uttranchal (formerly part of U.P.). In 2005, former Karnataka Chief Minister Bangarappa resigned from the BJP to join the Samajwadi Party. He successfully held on to his Lok Sabha seat from Shimoga under a new party ticket.

At present, the Samajwadi Party's main ally is the Rashtriya Lok Dal, a small party which is supporting its government in Uttar Pradesh. The Congress is also providing vital support to that government, and likewise, the Samajwadi Party is providing outside support to the minority Manmohan Singh government at the Centre. Despite this, there is deep-seated public enmity between the two parties. This was evident during the March 2006 political fall-out which followed the expulsion of Samajwadi Party parliamentarian Jaya Bachchan on grounds of holding an "office of profit." SP leaders blamed the move on the Congress high-command, and tensions have increased since then, although neither has revoked its support.

The SP tries to maintain equal distance to Congress and Bhartiya Janta Party. But its main rival in Uttar Pradesh is Mayawati's BSP (Bahujan Samaj Party) which has emerged as a major political force in the state. The BSP primarily focuses on Dalit and backward caste votes.

In the recently conducted 2007 Uttar Pradesh legislative elections, the SP won only 96 seats as opposed to 146 in the previous elections. As a result, Chief Minister Mulayam Singh Yadav had to resign, with his rival Mayawati, leader of the Bahujan Samaj party (which won a majority of 207 seats) was sworn in as the Chief Minister.