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 Limoges. He received his early education at home and in 1794 was sent to Paris to prepare for the École Polytechnique after his father was arrested, into which he was admitted at the end of 1797. Three years later he was transferred to the École des Ponts et Chaussées, and shortly afterwards was assigned to C. L. Berthollet. In 1802 he was appointed demonstrator to A. F. Fourcroy at the École Polytechnique, where subsequently (1809) he became professor of chemistry. 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 1808, he was the co-discoverer of boron.

In Paris a street and a hotel near the Sorbonne are named after him. Also his grave is at the famous cemetery Père Lachaise in Paris.