“King Arthasiddhi” is an 18th century Mongolian translation of a Tibetan Buddhist novel known in Tibet also as a popular drama. Its composition goes back to Indian avadanas and jatakas. Its language differs from the “Classical” written Mongolian of the 18th-century Buddhist xylographs and shows a marked influence of the underlying Chakhar dialect. This publication offers a thorough literary-historical and linguistic analysis with the annotated transcription and facsimile of the manuscript kept in the Copenhagen Royal Library. It contributes to the knowledge of Mongolian literature and its Indo-Tibetan connections and to a better understanding of the language and style of the translator Caqar gebsi Lubsang cültim, a noted man of letters.