Alexis Thérèse Petit (October 2, 1791 - June 21, 1820) was a French physicist. Petit is known for is work on the efficiencies of air- and steam-engines, published in 1818. His well-known discussions with the French physicist Sadi Carnot, founder of thermodynamics, may have stimulated Carnot in the development theories of thermodynamic efficiency in heat engines.

Petit was born in Vesoul, France. He is best known for the Dulong-Petit law for the specific heat capacity of metals, which he co-discovered with Pierre Louis Dulong in 1819.

Petit died in Paris, France.

* Petit, Aléxis