A modeling of wrinkled membranes based on projection operator is presented within the classical membrane theory. The total strains in wrinkled membranes are decomposed into elastic and zero-strain energy parts. A projection matrix that extracts the elastic parts from the total strains is derived. The resulting modified elasticity matrix that represents the stress-strain relations in wrinkled membranes is thus obtained as product of the classical elasticity matrix and the projection matrix. A numerical example is presented to demonstrate the accuracy of the proposed method.