This book constitutes the proceedings of the 21st International Conference on Web Engineering, ICWE 2021, which was supposed to be held in Biarritz, France, in May 2021. Due to the corona pandemic the conference changed to a virtual format.The total of 22 full and 13 short contributions presented in this volume were carefully reviewed and selected from 128 submissions. The book also contains 6 demonstration, 1 poster, 3 PhD, and 3 tutorial papers. The papers were organized in topical sections named: Semantic Web; social Web; Web modeling and engineering; Web big data and data analytics; Web mining and knowledge extraction; Web of Things; Web programming; Web user interfaces; PhD symposium; posters and demonstrations; and tutorials.