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 revelation which is closely intertwined with human reason and not limited to the theophany at Mount Sinai. A liberal strand of Judaism, it is characterized by little stress on ritual and personal observance, regarding Jewish law as non-binding and the individual Jew as autonomous, and by a great openness to external influences and progressive values.

The origins of Reform Judaism lie in mid-19th-century Germany, where Rabbi Abraham Geiger and his associates formulated its basic principles, attempting to harmonize Jewish tradition with modern sensibilities in the age of emancipation. Brought to America by German-born rabbis, the denomination gained prominence in the United States, flourishing from the 1860s to the 1930s in an era known as "Classical Reform". 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 acting in its name is one of the main channels for adherents to express their affiliation. The movement's most significant center is in North America.

Various regional branches exist, including the Union for Reform Judaism (URJ) in the United States and Canada, 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 within 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, about 1 million of whom are registered adult congregants, and the rest are unaffiliated but identify with the movement. This makes Reform the second-largest Jewish denomination worldwide, after Orthodox Judaism.

Its inherent pluralism and the importance it places on individual autonomy impedes any simplistic definition of Reform Judaism; its various strands regard Judaism throughout the ages as a religion that was derived from a process of constant evolution. They warrant and obligate further modifications and reject any fixed, permanent set of beliefs, laws or practices. A clear description of Reform Judaism became particularly challenging since the turn toward a policy that favored inclusiveness ("Big Tent" in the United States) over a coherent theology in the 1970s. This transition largely overlapped with what researchers termed the transition from "Classical" to "New" Reform Judaism in America, paralleled in the other, smaller branches of Reform across the world. The movement ceased stressing principles and core beliefs, focusing more on the personal spiritual experience and communal participation. This shift was not accompanied by a distinct new doctrine or by the abandonment of the former, but rather with ambiguity. The leadership allowed and encouraged a wide variety of positions, from selective adoption of halakhic observance to elements approaching religious humanism.

The declining importance of the theoretical foundation, in favour of pluralism and equivocalness, drew large crowds of newcomers. It also diversified Reform to a degree that made it hard to formulate a clear definition of it. Early and "Classical" Reform were characterized by a move away from traditional forms of Judaism combined with a coherent theology; "New Reform" sought, to a certain level, the reincorporation of many formerly discarded elements within the framework established during the "Classical" stage, though this very doctrinal basis became increasingly obfuscated.

Critics, like Rabbi Dana Evan Kaplan, warned that Reform became more of a Jewish activities club, a means to demonstrate some affinity to one's heritage in which even rabbinical students do not have to believe in any specific theology or engage in any particular practice, rather than a defined belief system.

In regard to God, the Reform movement has always officially maintained a theistic stance, affirming the belief in a personal God. Despite this official position, some voices among the spiritual leadership have approached religious and even secular humanism. This tendency has grown since the mid-20th century among both clergy and constituents, leading to broader, dimmer definitions of the concept.

Early Reform thinkers in Germany clung to this precept; the 1885 Pittsburgh Platform described the "One God... The God-Idea as taught in our sacred Scripture" as consecrating the Jewish people to be its priests. It was grounded on a wholly theistic understanding, although the term "God-idea" was excoriated by outside critics. So was the 1937 Columbus Declaration of Principles, which spoke of "One, living God who rules the world". Even the 1976 San Francisco Centenary Perspective, drafted at a time of great discord among Reform theologians, upheld "the affirmation of God... Challenges of modern culture have made a steady belief difficult for some. Nevertheless, we ground our lives, personally and communally, on God's reality." The 1999 Pittsburgh Statement of Principles declared the "reality and oneness of God". British Liberal Judaism affirms the "Jewish conception of God: One and indivisible, transcendent and immanent, Creator and Sustainer".