This three-volume set CCIS 3019-3021 constitutes the proceedings of the 21st International Conference on Information Processing and Management of Uncertainty in Knowledge-Based Systems, IPMU 2026, held in Rome, Italy, during June 15-19, 2026.The 111 full papers included in these volumes were carefully reviewed and selected from 254 submissions. They are organized into topical sections as follows:Part I: Aggregation theory; Imprecise probabilities; Knowledge representation and modelling; Statistical inference and data analysis.Part II: Robustness in economics and finance, Foundations of fuzzy sets;  Fuzzy and multivalued logic; Explainable AI and decision making.Part III: Preference modelling; Cooperative automated systems; Fuzzy control: methods and applications; Fuzzy implication functions; Data mining.