Pierre-Simon Girard (Caen, 4 November 1765 – Paris, 30 November 1836) was a French mathematician and engineer, who worked on fluids.

A prodigy who invented a water turbine at age 10, Girard worked as an engineer at the École Nationale des Ponts et Chaussées. He was in charge of planning and construction of the Amiens canal and the Ourcq canal. He collaborated with Gaspard de Prony on the Dictionnaire des Ponts et Chaussées (Dictionary of Bridges and Highways). He wrote works on fluids and on the strength of materials.