Yiddish orthography is the writing system used for the Yiddish language. It includes Yiddish spelling rules and the Hebrew script, which is used as the basis of a full vocalic alphabet. Letters that are silent or represent glottal stops in the Hebrew language are used as vowels in Yiddish. Other letters that can serve as both vowels and consonants are either read as appropriate to the context in which they appear, or are differentiated by diacritical marks derived from Hebrew nikkud, commonly referred to as "nekudot" or "pintalach" (literally "points" as those marks are mostly point-like signs). Additional phonetic distinctions between letters that share the same base character are also indicated by either pointing or adjacent placement of otherwise silent base characters. Several Yiddish points are not commonly used in any latter-day Hebrew context; others are used in a manner that is specific to Yiddish orthography. There is significant variation in the way this is applied in literary practice. There are also several differing approaches to the disambiguation of characters that can be used as either vowels or consonants.

Words of Aramaic and Hebrew origin are normally written in the traditional orthography of the source language—i.e., the orthography of these words, which is consonant-based, is generally preserved (Niborski 2012). All other Yiddish words are represented with phonemic orthography. Both forms can appear in a single word—for example, where a Yiddish affix is applied to a Hebrew stem. Yiddish diacritics may also be applied to words that are otherwise written entirely with traditional orthography.

In the early 20th century, for cultural and political reasons, efforts were initiated toward the development of a uniform Yiddish orthography. A specimen initial practice was described in detail by the Yiddish lexicographer Alexander Harkavy in a Treatise on Yiddish Reading, Orthography, and Dialectal Variations first published in 1898 together with his Yiddish–English Dictionary (Harkavy 1898). Additional illustrations of this variation are provided in source excerpts in Fishman 1981, which also contains a number of texts specifically about the need (pro and con) for a uniform orthography. A detailed chronology of the major events during this normative action, including rosters of conference participants, bibliographic references to the documents they produced, and summaries of their contents, is given in Yiddish in Schaechter 1999. There is a less detailed (but extensive nonetheless) English language review of this process in Estraikh 1999.

The first action formally undertaken by a government was in the Soviet Union in 1920, abolishing the separate etymological orthography for words of Semitic (i.e., Hebrew and Aramaic) origin. Under this reform, Hebrew words were respelled according to their Yiddish pronunciation rather than their etymological spelling. The reform also eliminated German-inspired orthographic conventions such as silent "hej" and "ajen" letters and redundant gemination of consonants.

A more comprehensive reform was developed in 1928 and finalized in 1932. It abolished the five special word-final allographs for the consonants [f], [x], [m], [n], and [ts], which had been a distinctive feature of the Hebrew-based Yiddish writing system; this change was widely reintroduced in 1961.^{[citation needed]} Additional changes included: elimination of the silent aleph in word-medial positions; differentiating between the prefixes "af" and "uf"; changing the spelling of "ba" for prefixes and prepositions; eliminating doubled consonants at morphological seams; and adopting a Russified spelling of international terms and proper names. The most radical proposals involved the Latinization of Yiddish, which had precedents in pre-revolutionary plans like Ludwig Zamenhof's 1909 blueprint. During the Soviet period, Latinization gained momentum between 1930 and 1933, coinciding with a broader campaign to Latinize writing systems of multiple Soviet languages. Nojekh Shulman published an article supporting a universal Roman alphabet in 1919, while in 1930 Moyshe Kamenshteyn [Wikidata] advocated for Latinization in the pedagogical magazine Ratnbildung. Isaac Zaretzki [Wikidata] initially opposed to Latinization, became one of its strongest proponents by 1930, arguing that the Roman alphabet was more international, ideologically closer to Communism, and would make it easier to eliminate Hebrew words from Yiddish. Proposals included a 26-letter Roman alphabet (V. Kolchinski) and a 28-letter version (Bentsion Grande). Though some songbooks were published using a Latin-based "Czech transcription" for musical notation, the Latinization initiative was ultimately shelved by 1934, when it began to be mentioned negatively in official publications.

The changes are both illustrated in the way the name of the author Sholem Aleichem is written. His own work uses the form שלום־עליכם but in Soviet publication this is respelled phonetically to שאָלעמ־אלײכעמ also dispensing with the separate final-form mem and using the initial/medial form instead. This can be seen, together with a respelling of the name of the protagonist of his Tevye der milkhiker, by comparing the title pages of that work in the U.S. and Soviet editions illustrated next to this paragraph. The Germanized מילכיגער (milkhiger) in the former exemplifies another widespread trend, daytshmerish, discussed further below.

The efforts preliminary to the 1920 reform, which took place in several countries — most notably in Poland with focus on a uniform school curriculum — resulted in other devices that were not implemented as a result of any governmental mandate. These were further considered during the 1930s by the Yidisher visnshaftlekher institut, YIVO (YIVO 1930). This led to the development of their תּקנות פֿון ייִדישן אויסלייג (takones fun yidishn oysleyg – "Rules of Yiddish Orthography"), also known as the "SYO" (Standard Yiddish Orthography) or the "YIVO Rules" (1st edition YIVO 1935, current edition SYO 1999). This has become the most frequently referenced such system in present-day use. Although it regularly figures in pedagogical contexts, it would be misleading to suggest that it is similarly dominant elsewhere. Other orthographies are frequently encountered in contemporary practice and are house standards for many publishers.

A useful review of this variation is provided in the Oxford University כלל־תקנות פון יידישן אויסלייג (klal takones fun yidishn oysleyg – "Standard Rules of Yiddish Orthography") (Oxford 1992 and available online), written in and codifying a more conventional orthography than the one put forward by YIVO. Differences in the systems can be seen simply by comparing the titles of the two documents but they differ more fundamentally in their approaches to the prescription and description of orthographic detail. The former treats orthographic variation as a positive attribute of the Yiddish literature and describes essential elements of that variation. The latter presents a uniform Yiddish orthography, based on observed practice but with proactive prescriptive intent. Strong difference of opinion about the relative merit of the two approaches has been a prominent aspect of the discussion from the outset and shows little sign of abating. Although the Yiddish alphabet as stated in the SYO is widely accepted as a baseline reference (with a few minor but frequently encountered variations), the spelling and phonetics of the YIVO system of romanized transliteration discussed below, remain subjects of particular contention. The intent of the SYO is not to describe the spectrum of traditional orthographic practice. The bulk of Yiddish literature predates the formulation of those rules and the discrepancies are significant.