The wave equation for elastic waves in piezoelectric crystals is found, including the third‐order terms in the expansion of the thermodynamic potential in the strain and electric field. The effective nonlinear parameters for a general crystal are obtained. In the general case these parameters depend on the second‐order and third‐order coefficients: nonlinear piezoelectric, electrostrictive, and linear electro‐optic. These coefficients are considered for piezoelectric and nonpiezoelectric crystals. Some estimates are given of the values of the nonlinear coefficients.