Reform Judaism, also known as Liberal Judaism or Progressive Judaism, is a major Jewish denomination that emphasizes the evolving nature of Judaism, the superiority of its ethical aspects to its ceremonial ones, and belief in a continuous search for truth and knowledge, which is closely intertwined with human reason and not limited to the theophany at Mount Sinai. A highly liberal strand of Judaism, it is characterized by lessened stress on ritual and personal observance, regarding halakha (Jewish law) as non-binding and the individual Jew as autonomous, and great openness to external influences and progressive values.

The origins of Reform Judaism lie in 19th-century Germany, where Rabbi Abraham Geiger and his associates formulated its early principles. Since the 1970s, the movement has adopted a policy of inclusiveness and acceptance, inviting as many as possible to partake in its communities rather than adhering to strict theoretical clarity. It is strongly identified with progressive and liberal agendas in political and social terms, mainly under the traditional Jewish rubric tikkun olam ("repairing of the world"). Tikkun olam is a central motto of Reform Judaism, and taking positive action for its own sake is one of the main channels for adherents to express their affiliation. The movement's most significant center today is in North America.

Various regional branches exist, including the Union for Reform Judaism (URJ) in the United States, the Movement for Reform Judaism (MRJ) and Liberal Judaism in the United Kingdom, the Israel Movement for Reform and Progressive Judaism (IMPJ) in Israel and the UJR-AmLat in Latin America; these are united under the banner of the international World Union for Progressive Judaism (WUPJ). Founded in 1926, the WUPJ estimates it represents at least 1.8 million people in 50 countries, just under 1 million of which are registered adult congregants, as well as many unaffiliated individuals who identify with the denomination. This makes it the second-largest Jewish denomination worldwide, after Orthodox Judaism.