William Zeitlin (ca. 1850 – 1921 in Leipzig) was a Russian scholar and bibliographer born at Homel, government (guberniya) of Moghilef, about the middle of the 19th century. He is known especially as the author of Kiryat Sefer, or Bibliotheca Hebraica Post-Mendelssohniana (Leipzig, 1891-95), a bibliographical dictionary of modern Hebrew literature from the beginning of Moses Mendelssohn's epoch until 1890. The compilation of this work occupied Zeitlin for twenty years. He made extensive use of Benjacob's Otzar ha-Sefarim and of Fürst's Bibliotheca Judaica, and visited Vilna and Warsaw, the centers of the Hebrew book market, as well as many university cities—as Königsberg, Berlin, Geneva, and Paris—from the libraries of which he gathered additional material for his work.
The Qiryat Sefer indexes not only works in book form, but also important periodical articles, biographical sketches, and scientific essays, in addition to giving biographical notes on several authors. Zeitlin had previously prepared an index of works written on the Hebrew calendar, in which he enumerates seventy-seven Hebrew works; this index was published by Chayyim Jonah Gurland in Yevreiski Kalendar (St. Petersburg, 1882). In the Zeit. für Hebr. Bibl. (ix.3-4) Zeitlin has published an alphabetical list of anagrams and pseudonyms of modern Hebrew writers; he was also a contributor to several Hebrew periodicals, writing mostly biographical articles.