A review of the study of the official language of the Office of the Medieval Moldavian Principality is presented in the article. The history of the study of the official language of medieval Moldavian diplomacy began in the first half of the 19th century. It is connected to the name of the Rusin Iu. Venelin who collected and commented on the Wallachian and Moldavian Literary Documents published in 1840. In 1905 the Russian Slavist I. Yatsimirskiy noted that the Wallachian and Moldavian documents written in Slavonic have a significant meaning in the study of the culture of the Principalities. While there are several thousands of such documents "the Wallachian are twice less than the Moldavian". The study of the Literary Documents and Acts of the Moldavian State was carried out by scholars from various countries-the largest of which are the works of prominent scholars of Russia/USSR, Austro-Hungary, Rumania, Moldavia and the Ukraine. The research of the public figure of the Carpatho-Russian Rebirth Yakov Golovatskiy, the legal expert and historian V.A. Ulyanitskiy, the philologist-Slavist Bessarabians A.A. Kochubinskiy, P.A. Syrku and A.I. Yatsimirskiy, the Czech historian K.I. Jireček, the Polish Slavist E. Kałużniacki, the Slovenian linguist F. Miklosich, the Rumanian researchers I. Bogdan, A. Rosetti, I. Barbulescu, P. Panaitescu, D. Bogdan, the Soviet philologist M.N. Sergievskiy, his Moldavian colleagues N.G. CorlǍtianu, N.D. Raevskiy and many others made a substantial contribution to the study of the language of the written sources of medieval Moldavia. The majority of the scholars held an opinion that the language of the Moldavian office was South- West Russian, that is, the language of the local Slavic population-the Rusins. Galician elements in the language were noted. Understandably to a lesser degree, there was an Old Bulgarian influence, and occasionally Moldavian lexicon was used. The author comes to a conclusion that it was the language of the Rusins, the native population of the Carpatho-Dniestrovian Lands, that was the official language of Moldavia from the moment of its formation in the second half of the 14th to the beginning of the18th centuries. This language had a great influence on the development of the Moldavian language. In the actual lexicon of contemporary Moldavian there are more than 2000 words of East Slavic origin. The Rusin influence is traced through all of the Moldavian diplomacy of the 14th-17th centuries in various governmental documents (plaintive, dedications, confirmations, diplomatic immunity, judicial, security) and other administrative and state documents. A further study of the culture and the official language of the medieval principality of Moldova, taking into account the interaction with the culture and language of the Rusins in the Carpatho- Dniestrovian Lands, can lead to new and interesting discoveries.