Lagrange's identity may refer to:

• Lagrange's identity, an algebraic identity
• Lagrange's identity (boundary value problem), an identity in calculus
• Lagrange's trigonometric identities, two trigonometric identities
• Lagrange's four-square theorem, a theorem from number theory
• Lagrange polynomial for theorems relating to numerical interpolation
• Euler–Lagrange equation of variational mechanics