Matthias Alexander Castrén (1813–1852), a great Finnish researcher and fieldworker, the first professor of the Finnish language, undertook a vast range of studies, geographically from Norwegian Lapland to Siberia and in subjects from linguistics to ethnology. His extensive work in the Russian north made him one of the principal figures of Siberian and Finno-Ugrian studies. Castrén's pioneering contributions in Turkology, Mongolian studies and archaeology are also noteworthy. He spent almost ten years on expeditions outside Finland and during his short life, he died at the age of 39, he managed to collect a vast amount of material, which is set out in 32 volumes of his manuscripts. However, his name is less known than are the names of some of his prominent friends. In English there are only few comments on his travels and work and these are in the context of the history of Finnish science in the 19th century. Published materials of Castrén's work include two volumes in Finnish translated from Swedish, six volumes in Swedish, and twelve volumes in German. Castrén was born in a period which is often regarded as a turning point in Finnish history and as a key period in the formation of the Finnish nation. Presenting an overview of different fields of Castrén's research, this article analyses his scientific contribution along two lines: in the context of the rising national awareness in Finland in the 19th century and of the mainstream developments in the scientific scene of this time. Based on Castrén's travel diaries, correspondence and lectures, the article seeks to contribute to understanding the historical aspects that shaped this great researcher situated as he was between three national traditions of his time: his writing was mostly in Swedish, his research activities were mainly carried out on behalf of the Russian Academy of Sciences, and yet he is most prized as a pioneer of Finnish linguistics, ethnography, archaeology and other disciplines.