The increasing proliferation of Internet-of-Things (IoT) devices in modern smart grid infrastructures presents both transformative potential and unprecedented security challenges. As these devices play critical roles in energy distribution, real-time monitoring, and demand response, ensuring secure authentication and data integrity across decentralized and heterogeneous networks becomes imperative. Traditional centralized authentication models suffer from scalability issues, single points of failure, and limited resilience against sophisticated cyber-attacks. In this context, blockchain technology emerges as a compelling solution to establish trust, transparency, and tamper-resistance in smart grid communication systems. This study proposes a blockchain-based secure authentication framework specifically designed for decentralized IoT environments within smart grid networks. The framework integrates lightweight cryptographic techniques with a permissioned blockchain ledger to manage identity validation, access control, and trust propagation among interconnected devices. By leveraging smart contracts, the system enables autonomous device registration, mutual authentication, and secure session key establishment without dependence on centralized authorities. Furthermore, the model employs a hierarchical consensus protocol optimized for energy efficiency and latency, addressing the computational limitations of resource-constrained IoT nodes. Performance evaluations using simulation environments demonstrate enhanced resistance against replay attacks, Sybil attacks, and man-in-the-middle threats, while maintaining minimal overhead in authentication time and energy consumption. The proposed framework offers a scalable and resilient solution to secure large-scale IoT deployments in critical smart grid applications. By decentralizing trust and automating authentication processes, this blockchain-enabled architecture paves the way for next-generation, cyber-resilient energy systems.