Sabbatai Zevi (Hebrew: שַׁבְּתַי צְבִי, romanized: Šabbəṯay Ṣəḇī, August 1, 1626 – c. September 17, 1676) was an Ottoman former Jewish mystic and rabbi from Smyrna who converted to Islam. His family were Romaniote Jews from Patras.

Active throughout the Ottoman Empire, Zevi claimed to be the long-awaited Jewish Messiah and founded the Sabbatean movement. Central to his teachings was the belief that during the Messianic Age, acts traditionally considered sinful would transform into righteous ones. This antinomian doctrine led Zevi and his followers to deliberately violate Jewish commandments, a controversial practice that later inspired movements like the Frankists.

Upon arriving in Constantinople in February 1666, Sabbatai was imprisoned on the order of the grand vizier Köprülüzade Fazıl Ahmed Pasha. In September of that same year, after being moved from different prisons around the capital to the imperial courts' seat in Adrianople (now Edirne), he was judged on accusations of fomenting sedition. Sabbatai was given the choice of death or conversion to Islam by the Grand Vizier representing Sultan Mehmed IV. He chose conversion, donning an Islamic turban from then on. The heads of the Ottoman state then rewarded him with a generous pension for complying with their political and religious plans. About 300 families who followed Sevi also converted to Islam and became known as the Dönme, Turkish for "converts".

Subsequently, the Ottomans banished him twice, first within Constantinople and when he was heard singing Psalms with Jews there, to a small town known today as Ulcinj in what is now Montenegro. He died in isolation.

Sabbatai Zevi was born in the Ottoman city of Smyrna, allegedly on Tisha B'Av, one of Judaism's fast days, during The Three Weeks in 1626. In Hebrew, Sabbatai means Saturn; in Jewish tradition, "the reign of Sabbatai," the highest planet, was often linked to the advent of the Messiah.

Zevi's family were Romaniote Jews from Patras. His father, Mordecai, was a poultry dealer in the Morea. During the Ottoman–Venetian wars, Smyrna became the center of Levantine trade, and Mordecai became the Smyrnan agent of an English trading house, achieving some wealth in the process. His sister, Orah Gadol, was a learned woman in her own right, well known among the Jews of Smyrna.

Following the prevailing Jewish custom of the time, Sabbatai's father had him study the Talmud. He attended a yeshiva under the chief rabbi of Smyrna, Joseph Escapa. Studies in halakha (Jewish law) did not appeal to him, but Zevi did attain proficiency in the Talmud. At the same time, he was fascinated by mysticism and Kabbalah and was influenced by Isaac Luria. Practical Kabbalah, whose devotees used asceticism to communicate with God and the angels to predict the future and perform miracles, was especially appealing to him. As well as Luria's writings, he read the Zohar and practiced asceticism and Kabbalistic purification exercises called tikkunim.

During the first half of the 17th century, millenarian ideas about the approach of the Messianic Age were popular. They included ideas about the redemption of the Jews and their return to the land of Israel with independent sovereignty. The apocalyptic year was identified by Christian authors as 1666, and millenarianism was widespread in England. This belief was so prevalent that Menasseh Ben Israel, in his letter to Oliver Cromwell and the Rump Parliament, appealed to it as a reason to readmit Jews into England, saying, "[T]he opinions of many Christians and mine do concur herein, that we both believe that the restoring time of our Nation into their native country is very near at hand."