Privacy-by-Design (PbD) has evolved from a regulatory concept into a foundational theory for enterprise information systems. With enterprises increasingly adopting cloud computing, artificial intelligence, big data analytics, and distributed architectures, embedding privacy into system design is both a moral obligation and a legal necessity. This paper develops a theoretical model of Privacy-by-Design for Enterprise Information Systems (PbD-EIS) by synthesizing regulatory frameworks (e.g., GDPR), privacy engineering methodologies, socio-technical theory, and organizational governance models. The conceptual model positions privacy as a multidimensional construct integrated across architecture, processes, technologies, and user behaviors. The paper provides detailed components of privacy governance, risk modeling, system architecture, identity management, and policy enforcement. The proposed PbD-EIS framework offers guidance for CIOs, system architects, developers, and policymakers seeking to embed privacy into enterprise digital ecosystems.