Blockchain security for IoT explores how decentralized ledger technology can address the critical challenges of trust, privacy, and resilience in large-scale, heterogeneous IoT ecosystems. Research in this field focuses on using blockchain to secure device authentication, data integrity, and access control, preventing threats such as spoofing, data tampering, and unauthorized access. Studies highlight lightweight consensus mechanisms, edge–blockchain integration, and off-chain storage solutions that make blockchain feasible for resource-constrained IoT devices. Smart contracts are investigated for automating secure machine-to-machine (M2M) communication, firmware updates, and service agreements, while cryptographic methods like zero-knowledge proofs and elliptic curve cryptography enhance privacy. Researchers also analyze scalability, energy efficiency, and latency concerns, proposing hybrid architectures that combine blockchain with edge or fog computing. Overall, papers in this area demonstrate how blockchain strengthens IoT networks by creating transparent, tamper-resistant, and secure infrastructures suitable for applications such as smart homes, connected vehicles, industrial IoT, and healthcare IoT.