This Study is designed to introduce the old Hangul manuscript Sire.L.13 kept in the Bibliotheca Apostolica Vaticana and to elucidate its significance. This manuscript is identified as a Korean translation of Ineffabilis Deus, an apostolic constitution promulgated by Pope Pius IX in 1854, which defined the dogma of the Immaculate Conception of the Blessed Virgin Mary. This study, as the first scholarly work on this manuscript, corroborates the significance of Sire.L.13 as follows. First, Marie-Dominique Sire, the principal of the Saint Sulpice Seminary in Paris, carried out the project to translate Ineffabilis Deus into “living” languages around the world and to dedicate these translations to Pope Pius IX. This old Hangul manuscript came to be one result of this worldwide translation project. Second, the letter of Sire requesting a Korean translation arrived in Choseon in February, 1863, but the Latin text of Ineffabilis Deus did not come until the end of June. Bishop Daveluy took charge of translation from the Latin text into Korean, and Korean believers transcribed the Korean text and decorated it with beautiful flower designs. All these works of translation, transcription, and decoration were completed before September 8. Bishop Berneux, Apostolic Vicar of Korea, mentioned this translation several times in November in his letters, and ratified it with his seal on November 25, sending it to France. This study demonstrates that Sire.L.13 was composed and dedicated to the pope through this process. Third, Sire.L.13 proves to be the first work which translated the whole text of a papal bull into Korean, as well as a rare old Hangul manuscript whose original text, translator, and the date of translation can be determined with great certainty. Besides, this manuscript demonstrates translation technique, vocabularies, usages, and notation adopted by Korean Catholics during the 1860s, and illustrates how they express into Korean the dogmatic definition of the immaculate conception of Mary, scriptural and patristic testimonies on Mary, and technical terms related to liturgies and rites and ecclesiastical offices. For this reason, Sire.L.13 will be an invaluable source material for the 19th century Latin-Korean glossary.