A Ritz approach for the analysis of buckling and postbuckling of stiffened composite panels with through-the-thickness cracks and/or delaminations is presented. The structure is modeled as the assembly of plate elements of which the behavior is described by the first-order shear deformation theory and von Kármán’s geometric nonlinearities. Penalty techniques ensure continuity along the edges of contiguous plate elements and the enforcement of the restraints on the external boundaries. They are also used to avoid interpenetration problems. General symmetric and unsymmetric stacking sequences are considered. A computer code has been developed and used to validate the proposed method, comparing the results with literature and finite-elements solutions. Results are also presented for postbuckling of multilayered stiffened plates with through-the-thickness cracks and delaminations. The analyses carried out show the efficiency and potential of the method, which provides accuracy comparable to those of other techniques in conjunction with a reduction in the number of degrees of freedom and simplification in data preparation.