Lehi (Hebrew pronunciation: [ˈleχi]; Hebrew: לח"י – לוחמי חרות ישראל Lohamei Herut Israel – Lehi, "Fighters for the Freedom of Israel – Lehi"), often known pejoratively as the Stern Gang, was a Zionist paramilitary militant organization founded by Avraham ("Yair") Stern in Mandatory Palestine. Its avowed aim was to evict the British authorities from Palestine by use of violence, allowing unrestricted immigration of Jews and the formation of a Jewish state. It was initially called the National Military Organization in Israel, upon being founded in August 1940, but was renamed Lehi one month later. The group referred to its members as terrorists and admitted to having carried out terrorist attacks.

Lehi split from the Irgun militant group in 1940 in order to continue fighting the British during World War II. It initially sought an alliance with Fascist Italy and Nazi Germany. Believing that Nazi Germany was a lesser enemy of the Jews than Britain, Lehi twice attempted to form an alliance with the Nazis, proposing a Jewish state based on "nationalist and totalitarian principles, and linked to the German Reich by an alliance". After Stern's death in 1942, the new leadership of Lehi began to move towards support for Joseph Stalin's Soviet Union and the ideology of National Bolshevism, which was considered an amalgam of both right and left. Regarding themselves as "revolutionary Socialists", the new Lehi developed a highly original ideology combining an "almost mystical" belief in Greater Israel with support for the Arab liberation struggle. This sophisticated ideology failed to gain public support and Lehi faired poorly in the first Israeli elections.

In April of 1948, Lehi and the Irgun were jointly responsible for the massacre in Deir Yassin of at least 107 Palestinian Arabs, including women and children. Lehi assassinated Lord Moyne, British Minister Resident in the Middle East, and made many other attacks on the British in Palestine. On 29 May 1948, the government of Israel, having inducted its activist members into the Israel Defense Forces, formally disbanded Lehi, though some of its members carried out one more terrorist act, the assassination of Folke Bernadotte some months later, an act condemned by Bernadotte's replacement as mediator, Ralph Bunche. After the assassination, the new Israeli government declared Lehi a terrorist organization, arresting some 200 members and convicting some of the leaders. Just before the first Israeli elections in January 1949, a general amnesty to Lehi members was granted by the government. In 1980, Israel instituted a military decoration, an "award for activity in the struggle for the establishment of Israel", the Lehi ribbon. Former Lehi leader Yitzhak Shamir became Prime Minister of Israel in 1983.