Claude Gaspard Bachet de Méziriac (October 9, 1581 – February 26, 1638) was a French mathematician born in Bourg-en-Bresse.

Bachet was a pupil of the Jesuit mathematician Jacques de Billy at the Jesuit College in Rheims. They became close friends.

Bachet wrote the Problèmes plaisants, of which the first edition was issued in 1612, a second and enlarged edition was brought out in 1624; this contains an interesting collection of arithmetical tricks and questions, many of which are quoted in W.W. Rouse Ball's Mathematical Recreations and Essays. He also wrote Les éléments arithmétiques, which exists in manuscript; and a translation, from Greek to Latin, of the Arithmetic of Diophantus (1621). It was this very translation in which Fermat wrote his famous margin note claiming that he had a proof of Fermat's last theorem.

Bachet was the earliest writer who discussed the solution of indeterminate equations by means of continued fractions. He also did work in number theory and found a method of constructing magic squares. Some credible sources also name him the founder of Bézout's identity.

For a year in 1601 Bachet was a member of the Jesuit Order. He lived a comfortable life in Bourg-en-Bresse and married in 1612. He was elected member of the French Academy in 1635.