Hasidism, sometimes spelled Chassidism, and also known as Hasidic Judaism (Ashkenazi Hebrew: חסידות Ḥăsīdus, [χasiˈdus]; originally, "piety"), is a Jewish religious group that arose as a spiritual revival movement in the territory of contemporary Western Ukraine during the 18th century, and spread rapidly throughout Eastern Europe. Today, most affiliates reside in Israel and the United States.

Israel Ben Eliezer, the "Baal Shem Tov", is regarded as its founding father, and his disciples developed and disseminated it. Present-day Hasidism is a sub-group within Haredi Judaism and is noted for its religious conservatism and social seclusion. Its members adhere closely both to Orthodox Jewish practice – with the movement's own unique emphases – and the traditions of Eastern European Jews. Many of the latter, including various special styles of dress and the use of the Yiddish language, are nowadays associated almost exclusively with Hasidism.

Hasidic thought draws heavily on Lurianic Kabbalah, and, to an extent, is a popularization of it. Teachings emphasize God's immanence in the universe, the need to cleave and be one with him at all times, the devotional aspect of religious practice, and the spiritual dimension of corporeality and mundane acts. Hasidim, the adherents of Hasidism, are organized in independent sects known as "courts" or dynasties, each headed by its own hereditary leader, a Rebbe. Reverence and submission to the Rebbe are key tenets, as he is considered a spiritual authority with whom the follower must bond to gain closeness to God. The various "courts" share basic convictions, but operate apart and possess unique traits and customs. Affiliation is often retained in families for generations, and being Hasidic is as much a sociological factor – entailing birth into a specific community and allegiance to a dynasty of Rebbes – as it is a purely religious one. There are several "courts" with many thousands of member households each, and hundreds of smaller ones. As of 2016, there were over 130,000 Hasidic households worldwide, about 5% of the global Jewish population.