A new exact closed-form solution based on Reddy’ third-order shear deformation plate theory for free vibration of thick circular/annular sector plates is presented. Characteristic equations are given for sector plates having hard simply supported radial edges and all combinations of free, soft simply supported, hard simply supported and clamped boundary conditions along the circular edges. Based on the present solution, the governing equations of the vibrated thick circular/annular sector plates were exactly solved by introducing the new auxiliary and potential functions as well as using the separation method of variables. To clarify the efficiency and accuracy of the present solution, several comparison studies are examined with the available data in the literature. Also, natural frequencies of the circular/annular sector plates are presented for different thickness–radius ratios, inner–outer radius ratios, sector angle values and boundary conditions.