Joseph Louis Gay Lussac (December 6, 1778–May 10, 1850) was a French chemist and physicist. He is known mostly for two laws related to gases.
Gay-Lussac was born at St Leonard, in the department of Haute-Vienne. He received his early education at home and in 1794 was sent to Paris to prepare for the Ecole Polytechnique, into which he was admitted at the end of 1797. Three years later he was transferred to the Ecole des Ponts et Chausses, and shortly afterwards was assigned to C. L. Berthollet. In 1802 he was appointed demonstrator to A. F. Fourcroy at the Ecole Polytechnique, where subsequently (1809) he became professor of chemistry, and from 1808 to 1832 he was professor of physics at the Sorbonne, a post which he only resigned for the chair of chemistry at the Jardin des Plantes. In 1831 he was elected to represent Haute Vienne in the chamber of deputies, and in 1839 he entered the chamber of peers.
In 1802, Gay-Lussac first formulated the law that a gas expands linearly with a fixed pressure and rising temperature (usually better known as Charles's Law).
In Paris a street and a hotel near the Sorbonne are named after him.