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The geometrically nonlinear effects in the out-of-plane flexural response of unreinforced masonry walls strengthened with externally bonded composite materials are analytically investigated. The investigation aims to explore the stabilizing or destabilizing influence of the arching action formed under realistic supporting conditions (restricted longitudinal deformations) of the wall, and to quantify the contribution of the strengthening system to improving the stability characteristics of the wall. The localized buckling effects associated with the development of compressive stresses in the FRP strip are also examined. Variational principles, large displacements kinematics, compatibility conditions between the structural components (masonry units, mortar joints, FRP strips, and adhesive layers), and the assumption of oneway flexural action are used for the formulation of the nonlinear analytical model. The cracking of the mortar joints, which is essential to the development of the arching action, and the formation of debonded zones are also considered. A numerical example that highlights the geometrically nonlinear effects in the response of the strengthened wall and examines the influence of the slenderness ratio is presented. The results quantify the potential increase of the limit point load and deflection due to the externally bonded composite system. They also quantitatively reveal the wrinkling phenomenon of the compressed FRP strip and the shear and peeling stress concentrations that develop in the vicinity of the cracked mortar joints, the debonded regions, and the wrinkled FRP layer. The paper closes with concluding remarks.