He was a Danish linguist and philologist who wrote several grammars and worked on comparative phonology and morphology. By the end of his life he knew twenty-five languages and dialects, and he is believed to have studied twice as many. His numerous philological manuscripts were transferred to the Royal Danish Library at Copenhagen.

Rask traveled extensively to study languages, first to Iceland, where he wrote the first grammar of Icelandic, and later to Russia, Persia, India, and Ceylon. Shortly before his death, he was hired as professor of Eastern languages at the University of Copenhagen. Rask is especially known for his contributions to comparative linguistics, being the first to show the relationship between the ancient Northern and the Western and Eastern Germanic languages, as well as to show their relationship with the Lithuanina, Slavonic, Greek and Latin languages.