Mobile Adhoc Networks (MANETs) function in dynamic, decentralized environments without fixed infrastructure, making them highly susceptible to security threats due to their ever-changing topology and limited resources. Conventional security approaches, which depend on centralized trust systems, face challenges in defending MANETs from attacks like blackhole, wormhole, and Sybil attacks. This paper examines how blockchain can serve as a decentralized, tamper-resistant security framework for MANETs. Essential blockchain elements, such as consensus protocols, cryptographic techniques, and distributed trust management, are evaluated for their effectiveness in reducing security risks. Additionally, the study reviews advanced blockchain methods, including trust based authentication, intrusion detection, and secure routing, assessing their benefits and constraints. It further explores open research challenges in terms of scalability, privacy, and energy optimization, offering perspectives on future advancements to strengthen blockchain- powered MANET security.