This book constitutes the refereed post-conference proceedings of the 9th EAI International Conference on IoT as a Service, IoTaaS 2023. The conference took place in Nanjing, China, during October 27-29, 2023. The 33 revised full papers were carefully reviewed and selected from 85 submissions. The papers present state-of-the-art research work on the challenges and developments related to IoT systems.