Marius Berthus Jansen (April 11, 1922 – December 10, 2000) was an American academic and historian. Born in Vleuten, Netherlands, his family moved to Rhode Island in 1923, and he grew up in Massachusetts. Jansen graduated from Princeton University in 1943, majoring in European history.
The same year, he began serving in the Army, where he studied Japanese and worked in the Occupation of Japan. He went on to complete his PhD in history at Harvard University in 1950, studying Japan with Edwin O.