Sifre (Hebrew: סִפְרֵי; siphrēy, Sifre, Sifrei, also, Sifre debe Rab or Sifre Rabbah) refers to either of two works of Midrash halakha, or classical Jewish legal biblical exegesis, based on the biblical books of Numbers and Deuteronomy.

The title Sifre debe Rav (lit. "the books of the school of Abba Arikha") is used by Chananel ben Chushiel, Isaac Alfasi, and Rashi; it occurs likewise in Makkot 9b.

The 8th century author of Halachot Gedolot names four "exegetical books belonging to the Scribes" (Heb. Midrash sofrim) and which, in all appearances, seem to refer to "Sifre debe Rav" and which comprised the following compositions: 1) Genesis Rabbah 2) Mekhilta of Rabbi Shimon ben Yochai (on Exodus), 3) Sifrei (on Numbers) and 4) Sifrei (on Deuteronomy).

Regarding the reference in Sanhedrin 86a to the Sifre of Rabbi Simeon, see Mekhilta of Rabbi Shimon ben Yochai; the question has likewise been raised whether, given the well-known close relation that existed between the school of Shimon bar Yochai and that of Rabbi Ishmael, the words וכלהו אליבא דר"ע apply to Rabbi Simeon's Sifre in the same degree as to the other works mentioned in this Talmudic passage.

Such questions, however, are unimportant in reference to the Sifre now extant; for this work is certainly not identical with the Talmudic Sifre; and, on closer investigation, it is found to be not a uniform work, but one composed of parts which did not originally belong together. Zecharias Frankel drew attention to the difference between that portion of the Sifre which refers to Numbers and that which refers to Deuteronomy, although, curiously enough, he misunderstood this difference and consequently arrived at false conclusions. David Hoffmann has correctly defined the relation between the two in his Zur Einleitung in die Halachischen Midraschim.

The Sifre to Numbers is evidently a midrash which originated in Rabbi Ishmael's school, and which has all the characteristics of such a work. It follows the same principles of exposition as does the Mekhilta; the same group of tannaim appears, and the same technical terms are employed. There are also many material points of similarity with the Mekhilta. The aggadic portions likewise contain many parallel passages.

It is especially noteworthy that the explanation in Sifre of the sotah law corresponds with a view expressed by Rabbi Ishmael (and also with the prescribed halakha), that one witness being sufficient to convict, the ordeal of the bitter water is not necessary. The explanation given in the Sifre to Numbers thus contradicts the explanation in Soṭah 31a and in Sifre, Deut. 188. The view expressed in the Babylonian Talmud is curious, for it cites the explanation of the Sifre to Numbers, and adds thereto: ואמר רחמנא תרי לית בה אלא חד והיא לא נתפשה אסורה‎, whereas the deduction should read to the contrary, תרי לית בה אלא חד היתה שותה‎. The Babylonian Talmud, which evidently does not know Rabbi Ishmael's view, tries to interpret the baraita in the sense of the prescribed halakha. But the baraita must in fact be interpreted in the opposite sense, namely, as following the view of Rabbi Ishmael, who, because עד always implies "two," as appears from Jerusalem Talmud Soṭah 20d, demands also in the case of a woman charged with adultery two witnesses of the alleged crime.

The passage introduced by the phrase סתם ספרי‎ (Sifre 161) = "an anonymous Sifre," likewise echoes Rabbi Ishmael's views; and the same is true of Sifre 21 as compared with Sifre 7. The beginning of Sifre 7 appears to be, strangely enough, an anonymous halakha expressing the opposite opinion, though this also may at need be harmonized with Rabbi Ishmael's view. Sifre 39 likewise follows Rabbi Ishmael's view, according to Hullin 49a. These and other less cogent reasons seem to indicate that the Sifre to Numbers originated in Rabbi Ishmael's school, although this does not exclude the assumption that the editor in addition borrowed much from Shimon bar Yochai's midrash and other less-known midrashim.