Simeon ben Zemah Duran, also Tzemach Duran (1361–1444) (Hebrew: שמעון בן צמח דוראן), known as Rashbatz (רשב"ץ) or Tashbatz was a Rabbinical authority, student of philosophy, astronomy, mathematics, and especially of medicine, which he practised for a number of years at Palma (de Majorca). A major 15th century posek, his published decisions in matters of halacha have been widely quoted in halachic literature for hundreds of years.
Simeon ben Tzemach was born in the Hebrew month of Adar, 1361. Various accounts put his birthplace as either Barcelona, or the island of Majorca. He was a near relation but not a grandson of Levi ben Gershon. He was a student of Ephraim Vidal, and of Jonah de Maestre, rabbi in Zaragoza or in Calatayud, whose daughter Bongoda he married.
After the persecution of 1391 (in the Balearic Isles) and Majorca he fled Spain with his father and sister for Algiers, where, in addition to practicing medicine, he continued his studies during the earlier part of his stay. In 1394 he and the Algerian rabbi Isaac ben Sheshet (known as Rivash) drafted statutes for the Jewish community of Algiers. After Rivash's retirement, Duran became rabbi of Algiers in 1407. Unlike his predecessor, he refused on principle to accept any confirmation of his appointment by the regent. As Duran had lost all his property during the massacre at Palma, he was forced against his will to accept a salary from the community, not having other means of subsistence. He held this office until his death. His epitaph, written by himself, has been reprinted for the first time, from a manuscript, in Orient, Lit. v. 452. According to Joseph Sambari, Simon was much respected in court circles (Medieval Jew. Chron. i. 130). He was the father of the Rashbash.