This book constitutes the refereed proceedings of the 21st International Symposium on Web and Wireless Geographical Information Systems, W2GIS 2024, held in Yverdon-les-Bains, Switzerland, during June 18-19, 2024. The 8 full papers and 7 short papers included in this book were carefully reviewed and selected from 20 submissions. The book also contains one invited talk. They were organized in topical sections as follows: Spatiotemporal Data Analysis, Open Data and Reproducible Research, Geospatial Technologies and Tools, Advanced Computing and GIS Applications, Transportation Applications, and Doctoral Symposium.