Thin-walled stiffened structures are widely employed as primary load-bearing components, owing to their high load-bearing capacity at low structural weight. Optimizing the stiffener layout can enhance the structural performance. In practical engineering applications, buckling is one of the primary failure modes of thin-walled structures; therefore, buckling capacity should be addressed alongside stiffness. Moreover, variations in load direction are another key factor affecting structural performance, indicating that robust stiffness should be incorporated into the design of thin-walled stiffened structures. To address these issues, design optimization of the stiffener layout for thin-walled structures is investigated. First, the stiffener layout optimization is realized for improving the stiffness performance of thin-walled stiffened structures. Then, to evaluate the buckling performance of thin-walled stiffened structures, a dual-thickness interpolation method is adopted to represent the thin-walled skin and stiffeners, and a universal shell element is adopted to compute the critical out-of-plane buckling load factor. By imposing a buckling constraint, the stiffener layout optimization is conducted for the stiffness design. Finally, to further account for variations in the load direction, a weighted sum of the expected value of compliance and its standard deviation is adopted as the objective function. Based on the above optimization framework, thin-walled stiffened structures with optimized stiffener layouts can be obtained. Numerical studies indicate that the thin-walled structures with optimized stiffener layouts exhibit lower compliance. Compared with stiffness-only designs, the obtained optimization results considering the load direction variation and the buckling constraint further reduce both the expected value and the standard deviation of compliance under load-direction uncertainty while maintaining a high out-of-plane buckling capability. In summary, the proposed optimization method for thin-walled stiffened structures can simultaneously achieve the robust stiffness performance and the high buckling load-carrying capacity.