Cybersecurity approach in sustainable smart cities is presented in this chapter to address vulnerabilities in digitally connected urban systems. Rapid integration of IoT and artificial intelligence increases risks such as data breaches, DoS attacks, ransomware, and compromised IoT connections. Blockchain and machine learning play a key role in securing infrastructures by ensuring tamperproof data records and transparent operations. A sustainable cybersecurity framework introduces zero-trust models, continuous threat monitoring, and secure IoT deployment for safeguarding citizen information. Strong security strategies enhance public trust, reduce operational disruptions, and protect against cyber-physical threats. Future research focuses on AI-driven security, scalable IoT protection, and privacy-preserving analytics.