Alexis Thérèse Petit (French: [pəti]; 2 October 1791, Vesoul, Haute-Saône – 21 June 1820, Paris) was a French physicist. Petit is known for his work on the efficiencies of air- and steam-engines, published in 1818 (Mémoire sur l’emploi du principe des forces vives dans le calcul des machines). His well-known discussions with the French physicist Sadi Carnot, founder of thermodynamics, may have stimulated Carnot in his reflexions on heat engines and thermodynamic efficiency. The Dulong–Petit law (1819) is named after him and his collaborator Pierre Louis Dulong.