"Transactions on Computational Collective Intelligence XXI" is a scholarly publication that focuses on computer-based methods of Computational Collective Intelligence (CCI) and their applications in various fields. These include, among others, the semantic web, social networks, and multi-agent systems. This edition, which is already the twenty-first, provides an in-depth insight into new methodological, theoretical, and practical aspects of CCI, understood as the intelligence that arises from the collaboration and competition of many individuals, both artificial and natural. The contributions in this volume are carefully selected and revised to reflect the latest developments and technologies in CCI. The application of various technologies of computer-based intelligence, such as fuzzy systems, evolutionary computations, and neural systems, is highlighted to support collective intelligence and create new forms of CCI in natural and artificial systems.