Jean Castaing (fl. 1685–1700) was a French engineer and inventor of the Castaing machine, a device used to add edge lettering to coins. Though edge lettering had existed for over a century, the earliest methods were costly and time-consuming. In 1649, Peter Blondeau introduced a new method at the Royal Mint in England, which he kept secret. Later, in 1685, Castaing proposed that his machine be used in the mints of France. His proposal was approved, and Castaing later became general manager of all the mints in the nation.
Castaing introduced other ideas, including the reforming and recoining of already existing French coins with edge lettering to reduce instances of counterfeiting and raise money for King Louis XIV's efforts in the Nine Years' War. During the carrying out of one such operation, Castaing was accused of malversation. His wife, Marie Hippolyte Castaing (née Bosch) petitioned the court on his behalf, and he was freed two years later. He died at some point in the early eighteenth century.