The Cathedral of Krk preserves two nearly identical “twin” antiphonals, likely dating to the mid-14th century, when the Diocese of Krk was under the jurisdiction of Grado. These manuscripts, written on parchment in a round Gothic script (Italian rotunda) and featuring black square notation, are extensive volumes containing the chants of the Divine Office for the entire liturgical year. This research highlights the previously unknown office of Saint Quirinus, the Bishop of Siscia, found in the antiphonals. Saint Quirinus has been celebrated as the patron saint of Krk at least since the 12th century, and according to some theories, as early as the 6th century. Given the direct reference to Krk within the text, it is certain that the antiphonals were specifically commissioned for use in the Diocese of Krk. Roughly half of the chants are based on well-known chants from the offices of Saint Anthony of Padua and Saint Francis, attributed to Julian of Speyer, while the source of the text is unknown. This study examines the origin and creation context of these codices by analysing the textual content, musical elements, and palaeographic and codicological features. Additionally, a full transcription of the Office of St. Quirinus is provided to support its future performance.