Jean Baptiste Joseph, chevalier Delambre (19 September 1749 – 19 August 1822) was a French mathematician, astronomer, historian of astronomy, and geodesist.

In 1790, to establish a universally accepted foundation for the definition of measures, the National Constituent Assembly asked the French Academy of Sciences to introduce a new unit of length.

The academics decided on the metre, defined as 1 / 10,000,000 of the distance from the North Pole to the equator, and prepared to organise an expedition to measure the length of the meridian arc between Dunkirk and Barcelona.

Cassini was chosen to head the northern expedition but, as a royalist, he refused to serve under the revolutionary government after the arrest of King Louis XVI on his Flight to Varennes.

On 15 February 1792, Delambre was elected unanimously a member of the French Academy of Sciences and in May 1792, after Cassini's final refusal, was placed in charge of the northern expedition, measuring the meridian from Dunkirk to Rodez in the south of France.