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.