The four-volume set constitutes the workshop proceedings of the 26th International Conference on Computational Science, ICCS 2026, which took place in Hamburg, Germany, during June 29 – July 1, 2026. The 133 full papers and 46 short papers presented in these proceedings were carefully reviewed and selected from 353 submissions. The papers are organized in the following topical sections:Volume I:Advances in High-Performance Computational Earth Sciences; Artificial Intelligence and High-Performance Computing forAdvanced Simulations; Artificial Intelligence for Network Analysis in Biology and Beyond; Biomedical and Bioinformatics Challenges for Computer Science; Computational Health; and Computational Methods for Petascale Research. Volume II:Computational Modeling and Artificial Intelligence for Social Systems; Computational Optimization, Modelling and Simulation; Computational Psychology and Mental Health; Computer Vision in the Era of Large Neural Models; Computing and Data Science for Materials Discovery and Design. Volume III:Credible Multiscale Modelling and Simulation; Large Language Models related Intelligent Decision-Making in the Digital Economy Era; Machine Learning and Data Assimilation for Dynamical Systems; Multi-Criteria Decision-Making: Methods, Applications, and Innovations; Navigating Trustworthy and Autonomous Modelling of Complex Systems: AI Meets Computational Science; Numerical Algorithms and Computer Arithmetic for Computational Science. Volume IV:Quantum Computing; Retrieval-Augmented Generation; Simulations of Flow and Transport: Modeling, Algorithms andComputation; Smart Systems: Bringing Together Computer Vision, Sensor Networks and Artificial Intelligence; Solving Problems with Uncertainties; Teaching Computational Science.