Rasmus Bartholin (/bɑːrˈtoʊlɪn, ˈbɑːrtəlɪn/; Latinized: Erasmus Bartholinus; 13 August 1625 – 4 November 1698) was a Danish physician and grammarian.

Bartholin was born in Roskilde. He was the son of Caspar Bartholin the Elder (1585–1629) and Anna Fincke, daughter of the mathematician Thomas Fincke.^[1]

As part of his studies, he travelled in Europe for ten years. He stayed in the Netherlands, England, France and Italy. In 1647, he took a Master's degree at the University of Copenhagen. In 1654, he received a Doctoral degree at the University of Padua.

He was a professor at the University of Copenhagen, first in Geometry, later in Medicine. He was also dean of the faculty of medicine, librarian, and rector.^[2] He wrote, in Latin, the first grammar of the Danish language, the 1657 De studio lingvæ danicæ.

Rasmus Bartholin is remembered especially for his discovery (1669) of the double refraction of a light ray by Iceland spar (calcite).^[3] He published an accurate description of the phenomenon, but since the physical nature of light was poorly understood at the time, he was unable to explain it.^[4] It was only after Thomas Young proposed the wave theory of light, c. 1801 that an explanation became possible.

He was a younger brother of Thomas Bartholin (1616–1680).^[5]