This book constitutes the refereed proceedings of the 9th International Symposium on Combinatorial Optimization, ISCO 2026, held in Kuşadası, Türkiye, during May 4–8, 2026.The 41 full papers were included in this were carefully reviewed and selected from 64 submissions. The papers cover the following topical sections: Scheduling; Mathematical Programming; Combinatorial Optimization; Polyhedral Combinatorics; Complexity; Approximation Algorithms; Network Design; Stochastic Optimization; Graphs and Combinatorics, and Applications.