Jacob ben Meir (1100 – 9 June 1171 (4 Tammuz)), best known as Rabbeinu Tam (Hebrew: רבינו תם), was one of the most renowned Ashkenazi Jewish rabbis and leading French Tosafists, a leading halakhic authority in his generation, and a grandson of Rashi. Known as "Rabbeinu" (our teacher), he acquired the Hebrew suffix "Tam" meaning straightforward; it was originally used in the Book of Genesis to describe his biblical namesake, Jacob.

Jacob ben Meir was born in the French country village of Ramerupt, now in Aube in northern-central France, to Meir ben Samuel and Yokheved, daughter of Rashi. His primary teachers were his father,and his brother Samuel ben Meir, known as Rashbam. His other brothers were Isaac, known as the Rivam, and Solomon the Grammarian. He married Miriam, the sister of Shimshon ben Yosef of Falaise, Calvados, who may have been his second wife.

His reputation as a legal scholar spread far beyond France. Abraham ibn Daud of the Taifa of Córdoba, a chronicler of the sages, mentioned Rabbeinu Tam in his Sefer ha-Qabbalah (but not Rashi). The Italian Mishnaic exegete Isaac ben Melchizedek of Siponto, maintained a written correspondence with Rabbeinu Tam. Rabbeinu Tam's work is also cited by Zerachiah ha-Levi of Girona, a Catalonian critic living in Lunel, Provence. He maintained a scholarly correspondence with Aaron ben Joseph of Beaugency and received questions from students throughout France and from the Italian communities of Bari and Otranto.

Rabbeinu Tam gave his Beth Din the title of "the generation's [most] significant court", and indeed, he is known for communal enactments improving Jewish family life, education, and women's status. At times, he criticised halakhic opponents, notably in his controversies with Meshullam of Melun and Efraim of Regensburg.

In or around 1160, a synod was held in Troyes as part of the Takkanot Shum. This synod was led by Rabbeinu Tam, his brother and his father, and Eliezer ben Nathan. Over 250 hakhams from communities all over France attended as well. Several communal decrees were enacted at the synod covering both Jewish-Gentile relations as well as matters relating internally to the Jewish community.

Legend has it that when Rashi was holding his infant grandson, the baby touched the tefillin on Rashi's head, and Rashi predicted that this grandson would later disagree with him about the order of the scripts that are put in the head tefillin. Rabbeinu Tam did disagree with Rashi and today, both "Rashi tefillin" and "Rabbeinu Tam tefillin" are produced: the Shulchan Aruch requires wearing Rashi's version and recommends that Jews wear both to satisfy both halakhic opinions.

It is worth noting that the Shulchan Aruch ... rules that Rabbeinu Tam Tefillin should be worn only by one who is known to be a very pious person; the Mishnah Berurah ... explains that it is a sign of haughtiness for anyone else to do this because the accepted practice is to wear Rashi Tefillin.

However, the Sephardic custom also followed in Hasidic Judaism is to wear Rabbeinu Tam's tefillin in addition to wearing Rashi's following the opinions presented in the Shulchan Aruch and its extensive commentaries authored throughout the early modern era and after. The rise and articulation of Hasidic Judaism has conflated the kabbalistic and halakhic aspects of Rabbeinu Tam's position, popularizing the custom to wear both pairs every day. Wearing Rabbeinu Tam tefillin is an almost universal custom among the many and diverse communities that follow the teachings of the Baal Shem Tov and his students.