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Jean Baptiste Joseph Delambre (September 19, 1749 in Amiens – August 19, 1822 in Paris) was a French mathematician and astronomer.

Delambre was born in Amiens, France.

After a childhood fever, Delambre suffered from very sensitive eyes and the belief that he would soon go blind. For fear of losing his ability to read, he devoured any book available to him and practised his ability to memorise. He thus immersed himself in Greek and Latin literature, acquired the ability to recall verbatum entire pages of books he may have read weeks beforehand, became fluent in Italian, English and German and even published Règles et méthodes faciles pour apprendre la langue anglaise (Rules and methods to easily learn English).

In order to establish a universally accepted foundation for the definition of the metre (to be defined as 1 / 10,000,000 of the distance from the North Pole to the equator), Delambre in collaboration with Pierre Méchain launched an expedition, lasting from 1792 to 1799, which measured the length of the meridian between Dunkerque and Barcelona. This portion of the meridian, which also passes through Paris, was to serve as the basis for the length of the quarter meridian, connecting the North Pole with the Equator.

Named director of the Paris Observatory and professor at the Collège de France, Delambre was one of the first astronomers to derive astronomical equations from analytical formulas, was the author of Delambre's Analogies and, after the age of 70, also the author of works on the history of astronomy like Histoire de l'astronomie.

Jean Baptiste Joseph Delambre died in 1822 and was interred in the Père Lachaise Cemetery in Paris.

Delambre crater on the Moon is named for him.

• O'Connor, John J., and Edmund F. Robertson. "Jean Baptiste Joseph Delambre". MacTutor History of Mathematics archive.
• http://www.surveyhistory.org/jean_baptiste_delambre.htm