Smart Cities utilise the latest technologies like sensors and cameras to maximise urban services and improve the living standards of citizens. The Internet of Things (IoT) makes an impactful contribution by gathering and processing data to enhance traffic management, energy efficiency, and public safety. However, the rapid expansion of smart city applications has created a need for strong and secure authentication mechanisms. This work identifies the shortfalls of the conventional centralised authentication systems and proposes a blockchain-based decentralised authentication scheme. By spreading the authentication process among various nodes, the suggested scheme improves security and resistance to cyber attacks. It uses both public and private blockchains and depends on smart contracts for safe and effective authentication of users and devices. The research discusses current authentication practices in smart city settings, defines the requirement for the decentralised model, and provides an overall design to address future challenges. In addition, it delves into the benefits of decentralised authentication in maintaining data privacy and integrity, making it a viable substitute for traditional models. This framework is anticipated to enable the scalable and secure interconnection of IoT devices, creating a good foundation for the evolution of urban infrastructure and emerging technologies in the future.