This book constitutes the proceedings of the 20th International Conference on Web Engineering, ICWE 2020, which was planned to take place in Helsinki, Finland, during June 9-12, 2020. Due to the corona pandemic the conference changed to a virtual format. The total of 24 full and 10 short contributions presented in this volume were carefully reviewed and selected from 78 submissions. The book also contains 4 PhD and 7 demo papers. The papers were organized in topical sections named: User interface technologies; performance of Web technologies; machine learning; testing of Web applications; emotion detection; location-aware applications; sentiment analysis; open data; liquid Web applications; Web-based learning; PhD symposium; demos and posters.