The period that goes from the end of the 18th century until the arrival of the first non-Iberian European immigrants, or their descendants, in the early 20th century, in the Northwest region of the state of Rio Grande do Sul (Brazil) is sparcely analyzed using the theory of agrarian systems. In this context, the present study aims to characterize the difference between the agrarian configurations and analyze the causes of the transformations that occurred in agriculture in that region, from the exclusively indigenous occupation until the period immediately before the modernization of agriculture, in the 1960s. Based on the theory of agrarian systems, the paper reconstitutes the evolution and differentiation of agriculture over that time, identifying the existence of three agrarian systems namely indigenous, caboclo and colonial. In addition to analyzing its configurations, the article discusses the causes of the decline of each of these systems.