Moses ben Nachman (Hebrew: מֹשֶׁה בֶּן־נָחְמָן Mōše ben-Nāḥmān, "Moses son of Nachman"; 1194–1270), commonly known as Nachmanides (/nækˈmænɪdiːz/; Greek: Ναχμανίδης Nakhmanídēs), and also referred to by the acronym Ramban (/ˌrɑːmˈbɑːn/; רמב״ן‎) and by the contemporary nickname Bonastruc ça Porta (Catalan: [ˌbɔnəsˈtɾuk sə ˈpɔrtə]; literally "Mazel Tov near the Gate", see astruc), was a leading medieval Jewish scholar, Catalan rabbi, philosopher, physician, kabbalist, and biblical commentator. He was raised, studied, and lived for most of his life in Girona, Catalonia. He is also considered to be an important figure in the re-establishment of the Jewish community in Jerusalem following its destruction by the Crusaders in 1099.

"Nachmanides" (Ναχμανίδης) is a Greek-influenced formation meaning "son of Nahman." He is also commonly known by the Hebrew acronym רמב״ן‎ (Ra-M-Ba-N, for Rabbeinu Mōšeh bēn-Nāḥmān, "Our Rabbi Moses son of Nahman"). His Catalan name was Bonastruc ça Porta (also written Saporta or de Porta), literally "Mazel Tov near the Gate."

Nachmanides was born in Girona in 1194, where he grew up and studied (hence he is also called Mosheh ben Nahman Gerondi, or "Moses son of Nahman the Gironan"), and died in the Land of Israel about 1270. He was a descendant of Isaac ben Reuben of Barcelona and cousin of Jonah Gerondi (Rabbeinu Yonah). Among his teachers in Talmud were Judah ben Yakar and Nathan ben Meïr of Trinquetaille, and he is said to have been instructed in Kabbalah (Jewish mysticism) by his countryman Azriel of Gerona, who was in turn a disciple of Isaac the Blind.

According to the responsa of Shlomo ibn Aderet Nachmanides studied medicine. During his teens he began to get a reputation as a learned Jewish scholar. At age 16, he began his writings on Jewish law. In his Milhamot Hashem (Wars of the Lord) he defended Alfasi's decisions against the criticisms of Zerachiah ha-Levi of Girona. These writings reveal a conservative tendency that distinguished his later works — an unbounded respect for the earlier authorities.

In the view of Nachmanides, the wisdom of the rabbis of the Mishnah and Talmud, as well as the Geonim (rabbis of the early medieval era) was unquestionable. Their words were to be neither doubted nor criticized. "We bow," he says, "before them, and even when the reason for their words is not quite evident to us, we submit to them" (Aseifat Zekkenim, commentary on Ketubot). Nachmanides' adherence to the words of the earlier authorities may be due to piety, or the influence of the northern French Jewish school of thought. However, it is thought that it also may be a reaction to the rapid acceptance of Greco-Arabic philosophy among the Jews of Spain and Provence; this occurred soon after the appearance of Maimonides' Guide for the Perplexed. This work gave rise to a tendency to allegorize Biblical narratives, and to downplay the role of miracles. Against this tendency Nachmanides strove, and went to the other extreme, not even allowing the utterances of the immediate disciples of the Geonim to be questioned.

Called upon, about 1238, for support by Solomon ben Abraham of Montpellier, who had been excommunicated by supporters of Maimonides, Nachmanides addressed a letter to the communities of Aragon, Navarre, and Castile, in which Solomon's adversaries were severely rebuked. However, the great respect he professed for Maimonides (though he did not share the latter's views), reinforced by innate gentleness of character, kept him from allying himself with the anti-Maimonist party and led him to assume the role of a conciliator. Maimonides was 58 years old when Nachmanides was born.

In a letter addressed to the French rabbis, he draws attention to the virtues of Maimonides and holds that Maimonides' Mishneh Torah – his Code of Jewish Law – not only shows no leniency in interpreting prohibitions within Jewish law, but may even be seen as more stringent, which in Nachmanides' eyes was a positive factor. As to Maimonides' Guide for the Perplexed, Nachmanides stated that it was intended not for those of unshaken belief, but for those who had been led astray by the non-Jewish philosophical works of Aristotle and Galen. (Note that Nachmanides's analysis of the Guide is not the consensus view of modern scholars.) "If," he says, "you were of the opinion that it was your duty to denounce the Guide as heretical, why does a portion of your flock recede from the decision as if it regretted the step? Is it right in such important matters to act capriciously, to applaud the one to-day and the other tomorrow?"

To reconcile the two parties, Nachmanides proposed that the ban against the philosophical portion of Maimonides's Code of Jewish law should be revoked, but that the ban against public study of the Guide for the Perplexed, and against those who rejected allegorical interpretation of the Bible, should be maintained and even strengthened.