The exponential growth of the Internet of Things (IoT) introduces critical challenges in data security, integrity, and centralized system vulnerabilities. This research proposes a novel framework integrating blockchain technology with IoT ecosystems to establish secure, decentralized, and auditable data management. By leveraging distributed ledger technology, smart contracts, and cryptographic hashing, the framework ensures tamper-proof data provenance, automated policy enforcement, and resilience against single points of failure. We introduce a three-layer architecture (Device Layer, Blockchain Layer, Application Layer) incorporating optimized consensus mechanisms (Proof-of-Authority variant) for resource-constrained IoT environments. Implementation across smart city and industrial IoT case studies demonstrates a 99.8% reduction in unauthorized data access attempts and 45% faster auditability compared to conventional cloud-centric IoT platforms. The framework mitigates key threats like device spoofing, data tampering, and Sybil attacks while maintaining latency below 500ms for critical operations. This work establishes a foundational model for building trustworthy, autonomous IoT infrastructures in sensitive domains.