This book constitutes the proceedings of the 21st International Conference on Soft Computing Models in Industrial and Environmental Applications, SOCO 2026, held in Marbella, Spain, during June 18-19, 2026.The 58 full papers included in this volume were carefully reviewed and selected from 121 submissions. The papers cover the following topical sections: Computer Vision Applications; Prediction and Monitoring; and Systems and Optimization. The papers also include the following special sessions: Efficiency and Explainability in Machine Learning and Soft Computing; Time Series Forecasting in Industrial and Environmental Applications; Quantum Computing; Intelligent Techniques Applied to Modelling and Control in Engineering focused on Marine Energy Systems and Robotics; Machine Learning and Computer Vision in Industry 4.0; Optimization, Modeling and Control by Soft Computing Techniques; EUropean Transnational Educational.