The rapid expansion of Internet-connected smart and edge devices has increased the exposure of digital systems to security threats, while also imposing strict constraints on computation, energy, and communication resources. Conventional security mechanisms are often unsuitable for such environments due to high latency, centralized control, and excessive overhead. Blockchain technology offers decentralized trust and tamper resistance, but its direct adoption in IoT and edge networks introduces scalability and performance challenges. To address these limitations, this paper presents the Blockchain-Integrated Lightweight Security Protocol (BILSP), a hybrid framework that combines blockchain-based authentication with lightweight cryptographic operations. BILSP employs elliptic curve cryptography and hash-based message authentication to enable secure device registration, mutual authentication, and session management, while smart contracts provide decentralized logging and access control. The proposed protocol is evaluated using NS-3, Contiki Cooja, and an Ethereum testnet under realistic IoT scenarios. Experimental results indicate that BILSP achieves reduced authentication delay, low energy consumption, and a high attack detection rate when compared with traditional and blockchain-only security approaches. These results demonstrate that BILSP provides an effective balance between security, efficiency, and scalability, making it suitable for resource-constrained IoT and edge computing environments.