This book constitutes the refereed proceedings of the 5th EAI International Conference on Smart Grid and Internet of Things, SGIoT 2021, held in TaiChung, Taiwan, in December 2021.The 9 regular papers and 4 short papers presented were carefully reviewed and selected from 57 submissions.The papers cover a broad range of topics in wireless sensor, vehicular ad hoc networks, security, deep learning and big data.The papers are organized in two subject areas: applications on internet of things, and communication security, big data, neural networks and machine learning.