Jean Nicolas Fortin (1750–1831) was a French maker of scientific instruments, born on 9 August 1750in Mouchy-la-Ville in Picardy.
Among his customers were such noted scientists as Lavoisier, for whom he made a precision balance, Gay-Lussac, François Arago and Pierre Dulong.
Fortin is chiefly remembered for his design of barometer, now called a Fortin barometer, which he introduced in about 1800.
In this, the mercury cistern has a glass portion through which the mercury exposed to the atmosphere can be seen, and an ivory needle which was made just to touch its mirror image in the mercury before the reading was taken.
Maps from a later 1795 edition of this work were used by Messier to show the location of his discoveries.On 15 May 1831 he died in Mouchy-le-Châtel (Oise).