Isaac (Isaak) Hirsch Weiss, also Eisik Hirsch Weiss (Hebrew: יצחק הירש ווייס) (9 February 1815 – 1 June 1905), was an Austrian Talmudist and historian of literature born at Groß Meseritsch, Habsburg Moravia.

After having received elementary instruction in Hebrew and Talmud in various chadorim of his native town, he entered, at the age of eight, the yeshiva of Moses Aaron Tichler (founded at Velké Meziříčí in 1822), where he studied Talmud for 5 years. He then studied at home under a tutor, and later in the yeshiva of Trebitsch, Moravia, under Ḥayyim Joseph Pollak, and in that of Eisenstadt, Hungary under Isaac Moses Perles, returning to his home town in 1837.

From an early age, Weiss began to study Talmud and rabbinics. He felt a keen desire for the pursuit of the secular sciences also, of which he was deprived in his youth, although he had been instructed in German by his private tutor. In some of the yeshibot which he attended instruction was given also in the Hebrew language and grammar, but that did not satisfy Weiss. It was for this reason that he changed from one yeshibah to another, hoping that he would ultimately find one in which his desire for learning would be satisfied. Influenced by Nachman Krochmal, by Rapoport, and by Leopold Zunz's Gottesdienstliche Vorträge, Weiss devoted part of his time to the study of religious philosophy. Talmudic studies, however, occupied the greater part of his time, and during the years that he spent in his parents' home he wrote several pamphlets containing novellæ on Talmudic tractates, as well as on the Shulḥan 'Aruk, Yoreh De'ah and Ḥoshen Mishpaṭ. He also kept up a correspondence with many distinguished rabbis, particularly Joseph Saul Nathanson, and contributed to Stern's Kokebe Yiẓḥaḳ and to Kobak's Jeschurun. To the former he contributed articles on general subjects, as well as verses and a number of biographies, among which that of Rab (Abba Arika) deserves special notice. In the Jeschurun he published several articles on the origin of prayer.

In 1858 Weiss settled in Vienna, where he became corrector for the press in the printing establishment of Samarski and Dittmarsch. Six years later (1864) he was appointed lecturer in the Beit ha-Midrash Academy founded by Adolf Jellinek, holding that position until his death. In Vienna, where Jellinek and other prominent Jewish scholars were congregated, Weiss found greater scope for his literary activity. He immediately turned his attention to a Vienna edition of the Talmud, and the notes with which he provided most of the tractates give evidence of his vast erudition. Then, at the request of Jacob Schlossberg, he wrote a compendium of the laws and observances relating to the ritual; this work, which was entitled Oraḥ la-Ẓaddiḳ, was published by Schlossberg at the beginning of the Seder Tefillat Ya'aḳob (Vienna, 1861). In the following year Weiss edited the Sifra with the commentary of Abraham ben David of Posquières; to this work he added a historical and linguistic introduction in nine chapters, and he provided the text with critical and exegetical notes entitled Masoret ha-Talmud, giving the variants of different manuscripts as well as an index showing the parallel passages in both Talmudim.

In 1864 Weiss took a prominent part in the Kompert trial, publishing a pamphlet entitled Neẓaḥ Yisrael in support of the testimony of Horowitz and Mannheimer with regard to the belief in the Messiah. This work called forth a reply by Nissan Schidhoff, entitled Nesheḳ Bar (Fürth, 1864). See Heinrich Graetz #The Kompert Affair.

In the same year Weiss edited the mishnayot of the treatise Berakhot, giving a list of variants in both Talmudim and a brief synopsis of the contents. A year later (1865) he founded a monthly magazine, Bet ha-Midrash, of which, however, only five numbers appeared. In the same year he edited the Mekilta, to which he added an introduction dealing with the historical development of both Halakha and Aggada, and a critical commentary entitled Middot Soferim.

After the publication of his Mishpaṭ Leshon ha-Mishnah (1867), an essay on the mishnaic language, Weiss began to prepare his stupendous work, the Dor Dor we-Dorshaw (1871–91; see below). Although Weiss had not been successful with his Bet ha-Midrash, he was more fortunate with the Bet Talmud, a monthly magazine which he founded in 1881 with Meïr Friedmann. In this periodical Weiss published numerous articles of his own, most of them treating of the Talmud in general and of Talmudic subjects. No less important are his biographical sketches, among which are those of Maimonides, Rashi, and Jacob Tam (Bet Talmud, i., ii., and iii., and reprinted in book form under the title Toledot Gedole Yisrael). In 1891, on the completion of his Dor, Weiss reedited Isaac Campanton's Darke ha-Gemara, a methodology of the Talmud. His last work in book form was his Zikronotai (Warsaw, 1895), a collection of his reminiscences from his childhood to his eightieth year.

He continued to contribute to various Hebrew periodicals, writing mostly biographies, of which may be mentioned that of Saadia Gaon (in Ha-Asif, ii. 275-293), published before Weiss had attained his thirtieth year, and that of Mannheimer (in Mi-Mizraḥ umi-Ma'Arab, iii. 17 et seq.). In his lectures Weiss was rather free with regard to the text of the Talmud and the Midrashim. He did not hesitate to declare the text faulty when it seemed so to him; but, on the other hand, he was very careful in making corrections. He held also that the words of the ancient rabbis should not be interpreted according to modern conception, such interpretation being liable to result in error.