Among the characteristic features of the Finnish language is the use of numerous derivational affixes and diverse word-formation options. Although Finnish has very old derivational elements, fairly recent suffixes and even completely new ways of forming words are also found. It is typical of word-formation options that they change, and that their frequency and popularity varies over time. In this diachronic study, the focus is on one of the most recent suffixes used in the Finnish language, the agentive -Ari suffix (e.g. kaahari ‘reckless driver’, kuohari ‘gelder of animals’). What makes the -Ari derivatives special is that the type has been adopted on the model of words borrowed from the Germanic languages. Historically these are descended from the Latin derivational element -ārius, which was adopted widely in the European languages. The main purpose of the present study was to find out whether, from a diachronic perspective, the -Ari-derived agent nouns actually represent an independent derived semantic category in Finnish. Another purpose was to characterize the process whereby the -Ari suffix was adopted in Finnish: at what point do these derived forms actually first occur in Finnish, and how has the use of the derivational element been manifested at different times. A final significant task of the study was to clarify the potential reasons and motivations for this morphological borrowing.