The ten-volume set LNCS 12949 - 12958 constitutes the proceedings of the 21st International Conference on Computational Science and Its Applications, ICCSA 2021, which was held in Cagliari, Italy, during September 13 - 16, 2021. The event was organized in a hybrid mode due to the Covid-19 pandemic. The 466 full and 18 short papers presented in these proceedings were carefully reviewed and selected from 1588 submissions. Part VIII of the set includes the proceedings of the following workshops: International Workshop on Privacy in the Cloud/Edge/IoT World (PCEIoT 2021); International Workshop on Processes, methods and tools towards RE-Silient cities and cultural heritage prone to SOD and ROD disasters (RES 2021); International Workshop on Risk, resilience and sustainability in the efficient management of water resources: approaches, tools, methodologies and multidisciplinary integrated applications (RRS 2021); International Workshop on Scientific Computing Infrastructure (SCI 2021); International Workshop on Smart Cities and User Data Management (SCIDAM 2021).