Smart cities can be made into super-smart cities through IoT devices’ implication of energy-efficient 6G. IoT devices are expected to reach fifty billion, but limited information is available regarding the energy-efficient 6G wireless communication standard. This article highlights the key technologies, applications, and trends in the Internet of Things (IoT) for energy-efficient 6G wireless communication in smart cities. The systematic review helped to achieve the aim of the study by considering the 20 articles extracted from databases and Google that fell between 2015 and 2021 and are written in English. The findings identified that quantum communication, blockchain, visible light communication (VLC), 6G brain−computer interface (BCI), symbiotic radio, and others are the key technologies. The applications of IoT technologies and energy-efficient 6G are found in 15 Minute City, Industrial Town, Intelligent Transport systems and others. Furthermore, the trend of using 6G through IoT devices in smart cities is promising.