The increasing deployment of Internet of Things (IoT) devices has significantly impacted the development and management of smart city infrastructures. These devices facilitate automation, real-time data collection, and efficient resource management across various sectors, including transportation, healthcare, energy, and environmental monitoring. However, integrating diverse IoT devices across vast networks has introduced a new array of security challenges, primarily associated with data transmission vulnerabilities. Secure data transmission in IoT networks is crucial to prevent data breaches, unauthorized access, and cyber-attacks that can compromise sensitive information, disrupt services, and threaten public safety. This paper presents a comprehensive approach to secure data transmission in IoT-based smart city networks, leveraging a multi-layered security model. The proposed solution combines advanced encryption techniques, secure key management protocols, and robust authentication mechanisms to ensure data confidentiality, integrity, and availability across IoT networks. We incorporate AES-256 encryption for efficient and high-speed data encryption, Public Key Infrastructure (PKI) for device authentication, and the Diffie-Hellman key exchange protocol for secure key management. Additionally, we utilize lightweight communication protocols like Message Queuing Telemetry Transport (MQTT) with Transport Layer Security (TLS) to maintain data integrity while minimizing computational overhead on resource-constrained devices.