This chapter applies a Marxist perspective to the Geographical Society of Rio de Janeiro (Sociedade de Geografia do Rio de Janeiro—SGRJ) between its founding in 1883 and Brazil’s transformation from an empire into a republic in 1889. While Brazil abolished slavery in 1888, it remained an oligarchy ruled by the senhores of the lordly estates. We argue that an institution such as the SGRJ, which was founded and run by such senhores, served as one of the tools that enabled them to modernise the country in, paradoxically, a conservative manner and thereby safeguard their power and privilege. By promoting the scientific exploration and mapping of the Brazilian territory, the SGRJ contributed to knowledge that helped both define the Brazilian nation-state and develop its economic potential. To maximise the latter, Brazil’s enormous natural resources were opened to the capitalist world economy led by Europe and the U.S., leaving Brazil as a semi-colony of the Global North. As a case in point, we discuss the Madeira-Mamoré Railway project, which aimed to facilitate transport through Brazil’s Amazon region between Bolivia and the Atlantic Ocean, and was eagerly supported by the SGRJ. In the late 1880s, geographical societies were founded in Brazil’s resource-rich areas themselves—the Amazon (Manaus) and Minas Gerais (Ouro Preto). We show that the SGRJ, through its Bulletin, entered into relations with these societies as well as with other Brazilian and foreign periodicals, especially those of foreign geographical societies. Consequently, many other influential institutions adopted the SGRJ’s knowledge and views on the Amazon.