This book constitutes the refereed proceedings of the 15th International Conference on Computational Collective Intelligence,  ICCCI 2023, held in Budapest, Hungary, during September 27–29, 2023.The 63 full papers included in this book were carefully reviewed and selected from 218 submissions. They are organized in topical sections as follows: collective intelligence and collective decision-making; deep learning techniques;  natural language processing; data mining and machine learning; social networks and intelligent systems; cybersecurity, blockchain technology and Internet of Things; cooperative strategies for decision making and optimization; computational intelligence for digital content understanding;  knowledge engineering and application for Industry 4.0; computational intelligence in medical applications; and ensemble models and data fusion.