<p>Determining the vibration characteristics of composite plates under hygrothermal environments is essential for accurate structural assessment. This study investigates the free vibration behavior of multilayer functionally graded carbon nanotube-reinforced composite (FG-CNTRC) laminated plates subjected to such conditions. Linear and nonlinear CNT distributions, arranged either uniformly or functionally graded across the ply thickness, are considered. Three laminated configurations are comparatively analyzed. The effective material properties of the carbon nanotube-reinforced composite (CNTRC) are determined using the extended rule of mixture, considering the temperature and moisture dependencies of the composite constituents. The first-order shear deformation theory (FSDT) is used to derive the coupled hygro-elastic, thermo-elastic relations, as well as the governing equations of the CNTRC plates. These equations are solved using the finite element method. A validation study is carried out to verify the accuracy of the employed approaches. Subsequently, a comprehensive parametric study examines the influence of hygrothermal conditions, plate geometry, CNT volume fraction, boundary conditions, CNT orientation, distribution uncertainty, and ply number on the vibrational response of FG-CNTRC laminated plates is conducted. Numeric findings suggest that changes in hygrothermal conditions and geometric parameters strongly impact the free vibration characteristics of CNTRC laminated plates. Given the remarkable findings outlined in this study, new ideas are introduced to offer innovative perspectives and original design frameworks for prospective practical applications in polymeric nanocomposite structures.</p>