The paradigm shift towards highly distributed renewable energy integration has exponentially increased the topological complexity of Smart Grids. Consequently, the tight coupling between operational and information networks exposes these systems to severe cyber threats, including data breaches and malicious intrusions. Conventional centralized dispatch paradigms struggle with delayed responses, suboptimal coordination, and opaque design lifecycles. To overcome these limitations, this study introduces an innovative Multi-Agent System architecture engineered via Model-Based Systems Engineering methodologies. By employing SysML, we established a comprehensive digital twin encompassing system requirements, functional layouts, and logical boundaries. The proposed framework deploys a decentralized hierarchy of four specialized agents—perception, decision making, execution, and collaboration—to execute collaborative defense protocols strictly bounded by electrical safety constraints. Validation through IEEE 33-node distribution network simulations confirms that the framework rapidly identifies and mitigates Denial of Service, data falsification, and unauthorized device access. This MBSE-MAS paradigm demonstrates exceptional scalability and resilience, offering a highly practical blueprint for safeguarding next-generation power infrastructure.