Mathieu Tillet (10 November 1714 Bordeaux - 13 December 1791) was a French botanist, agronomist, metallurgist and administrator.
He was the son of the goldsmith Gabriel Tillet and began studying metals at his father's workshop. In 1740 he was appointed Director of the Mint at Troyes. Ten years later he published his first book, about alloys. In 1750, he was awarded a prize by the Academy of Bordeaux for his research concerning the plagues of cereals, especially wheat. In 1755, he published the results of his research as Dissertation sur la cause qui corrompt et noircit les grains de blé dans les épis; et sur les moyens de prévenir ces accidents (Explanation of the cause that corrupts and blackens the grains of wheat in the ears; and the means to prevent these accidents), and was awarded another prize for it. The fungus Tilletia tritici, which he describes in this work, was named after him a century later by Charles and Louis Tulasne. In 1756 he resigned from his post at the Mint at Troyes and moved to Paris, living on Rue du Cloître-Notre-Dame.
On 9 September 1758, he was admitted to the French Academy of Sciences as a botanist. He occupied several offices at the Academy over the next decades, finally becoming Treasurer in 1788, after the death of his predecessor Buffon.
In 1760 and 1761 he traveled around the Angoumois, researching yet another cereal plague together with his friend and fellow agronomist Henri-Louis Duhamel du Monceau. The result of this was the book Histoire d'un insecte qui devore les grains de l'Angoumois (History of an Insect that Devours the Grains of the Angoumois, published by H. L. Guérin & L. F. Delatour, Paris, 1762).
In 1766 he was charged with the making of 24 copies of the Toise de l'Academie, the official measuring unit of length in the Kingdom of France, and their distribution to the main French cities of the time.
In 1784 he became the General Administrator of the Salpêtrière. It was during his administration that the Comtesse de la Motte was imprisoned there for her participation in the Affair of the diamond necklace, and escaped.
He was a member of the Metric Commission of the Academy of Sciences, with Jean-Charles de Borda, Joseph Louis Lagrange, Antoine Lavoisier, Marquis de Condorcet, Pierre-Simon Laplace and Gaspard Monge, who presented on 19 March 1791, a report to the National Constituent Assembly in which the basis for the length of the meter was established. The report rejected the fixing by a pendulum that swings once at the equator, and defined the length of the meter as 1 ten-millionth of a quarter of a meridian. The meridian was subsequently measured by Jean Baptiste Joseph Delambre and Pierre Méchain
- Alexander Lernet-Holenia: Das Halsband der Königin (The Queen's Necklace, Paul Zsolnay Verlag, Hamburg/Vienna, 1962, historical study on the affair of the diamond necklace, with a chapter on the life of Mathieu Tillet, and describing his role in the escape of the Comtesse de la Motte from prison)
-  Short bio in French