Rabbi Isaac ben Mordecai of Regensburg (Hebrew: יצחק בן מרדכי מרגנסבורג) also known by his acronym Ribam (Rabbi Isaac Ben Mordecai) was a 12th century German Tosafist.
Born in Prague, Bohemia, in his early years, Rabbi Isaac studied in Regensburg under Rabbeinu Tam and Isaac ben Asher ha-Levi.
In the following years, he served as the head of the city's bet din.
A large number of his tosafot are on the tractate Bava Batra, which are included in the first printed edition of the Talmud.
He also known to have written tosafot on tractates Pesaḥim, Mo'ed Katan, Bava Kamma, Shabbat, Ketubbot, Gittin, Sotah, Nazir, and Bava Meẓia.