Ole Christensen Rømer (Danish: [ˈoːlə ˈʁœˀmɐ]; 25 September 1644 – 19 September 1710) was a Danish astronomer who, in 1676, first demonstrated that light travels at a finite speed. Rømer also invented the modern thermometer showing the temperature between two fixed points, namely the points at which water boils and freezes.

Rømer made his discovery regarding the speed of light while working at the Royal Observatory in Paris and studying Jupiter's moon Io. He estimated that light takes about 11 minutes to travel from the Sun to Earth. Using today's knowledge of the Sun-Earth distance, this would amount to a speed of light of approximately 220,000 kilometers per second, compared to today's accepted value of just under 300,000 kilometers per second.

In scientific literature, alternative spellings such as "Roemer", "Römer", or "Romer" are common.

Rømer was born on 25 September 1644 in Aarhus to merchant and skipper Christen Pedersen (died 1663), and Anna Olufsdatter Storm (c. 1610 – 1690), daughter of a well-to-do alderman. Since 1642, Christen Pedersen had taken to using the name Rømer, which means that he was from the Danish island of Rømø, to distinguish himself from a couple of other people named Christen Pedersen. There are few records of Ole Rømer before 1662, when he graduated from the old Aarhus Katedralskole (the Cathedral school of Aarhus), moved to Copenhagen and matriculated at the University of Copenhagen. His mentor at the university was Rasmus Bartholin, who published his discovery of the double refraction of a light ray by Iceland spar (a transparent form of the mineral calcite) in 1668, while Rømer was living in his home. Rømer was given every opportunity to learn mathematics and astronomy using Tycho Brahe's astronomical observations, as Bartholin had been given the task of preparing them for publication.

Rømer was employed by the French government: Louis XIV made him tutor for the Dauphin, and he also took part in the construction of the magnificent fountains at Versailles.

In 1681, Rømer returned to Denmark and was appointed professor of astronomy at the University of Copenhagen, and the same year he married Anne Marie Bartholin, the daughter of Rasmus Bartholin. He was active also as an observer, both at the University Observatory at Rundetårn and in his home, using improved instruments of his own construction. Unfortunately, his observations have not survived: they were lost in the great Copenhagen Fire of 1728. However, a former assistant (and later an astronomer in his own right), Peder Horrebow, loyally described and wrote about Rømer's observations.

In Rømer's position as royal mathematician, he introduced the first national system for weights and measures in Denmark on 1 May 1683. Initially based on the Rhine foot, a more accurate national standard was adopted in 1698. Later measurements of the standards fabricated for length and volume show an excellent degree of accuracy. His goal was to achieve a definition based on astronomical constants, using a pendulum. This would happen after his death as practicalities made it too inaccurate at the time. Notable is also his definition of the new Danish mile of 24,000 Danish feet (circa 7,532 m).

In 1700, Rømer persuaded the king to introduce the Gregorian calendar in Denmark and Norway – something Tycho Brahe had argued for in vain a hundred years earlier.