This book constitutes the refereed proceedings of the 21st International Conference on Mobile Web and Intelligent Information Systems, MobiWIS 2025, held in Istanbul, Türkiye, during August 11–13, 2025. The 23 full papers and one short paper included in this book were carefully reviewed and selected from 75 submissions. They were organized in topical sections as follows: Mobile Web and Enterprises, Blockchain Technologies, Security and Privacy, Advanced AI Approaches, Mobile Systems and Applications, and Networking and SDN.