The Sanhedrin (Hebrew and Middle Aramaic סַנְהֶדְרִין, a loanword from Koine Greek: Συνέδριον, romanized: synedrion, 'assembly,' 'sitting together,' hence 'assembly' or 'council') was a Jewish legislative and judicial assembly of either 23 or 70 elders, existing at both a local and central level in the ancient land of Israel.

There were two classes of Rabbinite courts called sanhedrins: Greater and Lesser. A lesser Sanhedrin of 23 judges was appointed to sit as a tribunal in each city. There was only one Great Sanhedrin of 70 judges, which, among other roles, acted as a supreme court, taking appeals from cases that lesser courts decided. In general usage, the Sanhedrin without qualifier usually refers to the Great Sanhedrin, which was presided over by the Nasi, who functioned as its head or representing president, and was a member of the court; the Av Beit Din or the chief of the court, who was second to the Nasi and 69 general members.

In the Second Temple period, the Great Sanhedrin met in the Temple in Jerusalem, in a building called the Hall of Hewn Stones. The Great Sanhedrin convened every day except festivals and the sabbath day (Shabbat).

After the destruction of the Second Temple and the failure of the Bar Kokhba revolt, the Great Sanhedrin moved to Galilee, which became part of the Roman province of Syria Palaestina. In this period, the Sanhedrin was sometimes called the Galilean Patriarchate or Patriarchate of Palaestina, the governing legal body of Galilean Jewry. In the late 200s CE, to avoid persecution, the name Sanhedrin was dropped and its decisions were issued under the name of Beit HaMidrash (house of learning). The last universally binding decision of the Great Sanhedrin appeared in 358 when the Hebrew calendar was established. The Great Sanhedrin was finally disbanded in 425.

Over the centuries, attempts have been made to revive the institution, such as the Grand Sanhedrin convened by Napoleon Bonaparte and modern attempts in Israel.

In the Hebrew Bible, Moses and the Israelites were commanded by God to establish courts of judges. They were also commanded to establish a "supreme court" located at the central sanctuary (after arriving in the Land of Israel), to handle cases too difficult for local courts.

When Moses declared that the task of leading the people was too difficult for him, God had him appoint 70 elders (zekenim) to share the burden of leadership with him. According to the Mishnah, these 70 elders plus Moses himself are the source for the 71 judges of the "Great Sanhedrin". These elders are described as "the elders of the people and its officers", according to a midrash, they were the same officers who were beaten in Egyptian slavery for failing to meet Pharaoh's quota of bricks, and after the Exodus were rewarded with membership on the first Sanhedrin.

The 23 judges of the "Lesser Sanhedrin" are derived from the following exegesis: it must be possible for a "community" to vote for both conviction and exoneration (Numbers 35:24–25). The minimum size of a "community" is 10 men, thus 10 vs 10. One more is required to achieve a majority (11 vs. 10), but a simple majority cannot convict (Exodus 23:2), and so an additional judge is required (12 vs. 10). Finally, a court should have an odd number of judges to prevent deadlocks; thus 23 rather than 22.