<p>Internet of Things (IoT) is a rapidly growing domain encompassing various industries, applications, and technologies. Although data remains secure in encrypted form during transmission, managing individual keys for pairwise communication in large IoT networks is a challenging and complex task. A group key can enable effective message delivery by reducing key management overhead. Several proposed frameworks for secure group key management exist in literature but scalability challenges still remain. In particular, they become more significant during the join or leave operation phase which results in group-wide rekeying causing unnecessary computational overhead (proportional to number of users involved) and reduced network scalability. Additionally in few existing protocols, there is a threat to backward secrecy as the re-keying process is not secure; the already left devices can quickly recalculate the new group key. This work proposes a blockchain-based scalable framework that addresses these problems and maintains a consistent group key in dynamic IoT environments. The proposed framework utilises a highly efficient and secure Group Key Management protocol, ensuring both forward and backward secrecy. The scalability is achieved by limiting the initial group key setup phase to 2N-2 steps and only 1 step for adding or removing a user. The robustness to various kinds of attacks like sybil attack, replay attack, collusion attack and insider attack is demonstrated. Additionally, the security of the proposed framework is formally analysed using Burrows-Abadi-Needham logic. The performance of the proposed framework is evaluated through Network Simulator ‘NS2’ to show that the time cost is feasible and effective for IoT networks.</p>