Blockchain security for smart city research focuses on leveraging decentralized ledger technology to build secure, transparent, and resilient infrastructures that support urban services and governance. Research papers in this domain explore applications such as secure data sharing across IoT-enabled smart devices, decentralized identity management for citizens, tamper-proof e-governance records, transparent voting systems, and secure financial transactions for public services. Blockchain is also investigated for enhancing security in smart grids, intelligent transportation, healthcare, and waste or water management systems within cities. Studies highlight how smart contracts can automate service delivery, resource allocation, and policy enforcement, reducing human error and corruption risks. At the same time, challenges such as interoperability among heterogeneous city systems, scalability to handle massive data volumes, compliance with regulations, and privacy preservation are widely discussed. Proposed solutions include consortium blockchains, lightweight consensus mechanisms, and integration with edge/fog computing. Overall, this body of research emphasizes blockchain’s role in providing a secure, trustworthy, and citizen-centric foundation for future smart city ecosystems.