A so far unavailable quasi-3D trigonometric shear deformation theory for the bending analysis of functionally graded plates is presented. This theory considers the thickness-stretching effect ((Formula presented.)) by modeling the displacement field with just four unknowns and rich trigonometric shear strain shape functions. The principle of virtual works is used to derive the governing equations and boundary conditions. Results from this theory are compared with the CPT, first-order shear deformation theory (FSDT), and other quasi-3D HSDTs. In conclusion, this theory is more accurate than the CPT and FSDT and behaves as well as quasi-3D HSDTs having much less number of unknowns.