This book constitutes the refereed proceedings of the 20th International Conference and Workshops on Algorithms and Computation, WALCOM 2026, held in Perugia, Italy, on March 4–6, 2026. The 38 full papers were selected from carefully reviewed 108 submissions. WALCOM 2026 were orgainzed in following topical sections as Graph drawings and embeddings, Approximation, Induced subgraphs and dominating sets, Complexity, Games and graph reconfiguration, Shortest paths and minimum spanning trees, Geometric problems and Enumeration problems.