This book constitutes the refereed proceedings of the 21st International Conference on Web Information Systems and Applications, WISA 2024, held in Yinchuan, China, during August 2–4, 2024. The 39 full papers and 11 short papers presented in this book were carefully selected and reviewed from 193 submissions. These papers have been organized in the following topical sections: Knowledge construction; Intelligent service; Intelligent computing; Large language model; Security; Information system applications.