Carl Johan Sverdrup Marstrander (26 November 1883 - 23 December 1965) was a Norwegian linguist, known for his work on the Irish language. His works, largely written in Norwegian, on the Celtic and Norse components in Norwegian culture, are considered important for modern Norway.
He was a student of Sophus Bugge and Alf Torp, and spent time in Ireland from 1907, studying modern Irish on Great Blasket Island unerTomás Ó Criomhthain, and then teaching at the School of Irish Learning in 1910. He jointly edited Ériu volumes 5~6 (1911–12) with Kuno Meyer. From 1913 to 1954 he was Professor in Celtic languages at the University of Oslo. During the German occupation of Norway he was jailed several times, and once came close to execution after an arrest by the Gestapo. He influenced later linguists, including Alf Sommerfelt and Carl H. J. Borgstrøm.
He was general editor from 1910–14 for the long-projected historical Dictionary of the Irish Language, the first fasciculus of which was published by the Royal Irish Academy in 1913. His articles were of enduring influence, and published in Revue Celtique and Zeitschrift für Celtische Philologie, and his own journal, Norsk Tidsskrift for Sprogvidenskap which he founded in 1928.
His Bidrag til det Norske Sprogs historie i Irland (1915) and Les présents indo-européens à nasale infixée en celtique (1924), are two of his larger works.
He is also known for his writings on the history of the Isle of Man,  and for securing support and recognition for the Manx historian J. J. Kneen. He made pioneering sound recordings of the Manx language, at a time when few fluent native speakers survived.
- Binchy (1966), pp. 237–8.
- Diarmuid Ó Giolláin, Locating Irish Folklore: Tradition, Modernity, Identity (2000), pp. 125.
- Oskar Bandle, Kurt Braunmüller. The Nordic Languages: an international handbook of the history of the North Germanic languages (2002), p. 130.
- Professor Marstrander's contribution to Manx History