This article deals with the history of the arabisms in the Old Ruthenian language. The process related with lexical loanwords from the Arabic language dates back to the beginning of the written word in the Eastern Slavic region. From 11th century until the end of the 17th century 97 loanwords of Arabic etymology got transited into the Old Ruthenian language. The most numerous group of loanwords is the religious lexis and, related to Islam, the administrative lexis. The analysis of loaned arabisms proved that while passing to the Old Ruthenian language at first and then to the Russian language, they underwent adaptation processes of different kind (phonetic, morphologic, semantic, etc.). The majority of the arabisms considered here enriched the vocabulary of the Old Ruthenian language and they function in the Russian language until the present day.