Analytical solution taking into account the transverse normal strain is developed for the analysis of the mechanical and hygrothermal behaviors of functionally graded material (FGM) plates. In order to include the transverse normal deformation, the hygrothermal deformation through the thickness due to thermal and moisture loads is introduced in the transverse displacement field. Employing the bounding surface free traction condition, effects of transverse normal hygrothermal deformation can be incorporated in the in-plane displacement field. Moreover, there are only five displacement variables in the proposed model as transverse normal deformations due to hygrothermal loads are absorbed in the generalized force vector.By analyzing the hygrothermal behaviors of FGM plates, the accuracy and efficiency of the proposed model are assessed. Moreover, the effects of temperature, moisture concentration, transverse normal strain, volume fraction distributions and side-to-thickness ratios on the displacements and the stresses of FGM plates have all been studied.