Digital inclusion has become a foundational pillar of modern socio-economic development. As governments, markets, and institutions transition toward digital ecosystems, equitable access to digital technologies determines participation in education, finance, healthcare, governance, civic engagement, and employment. This paper presents a unified Digital Inclusion Theory that conceptualizes how access, affordability, literacy, infrastructure, policy, and socio-cultural factors shape developmental outcomes. The proposed Four-Dimensional Digital Inclusion Model bridges digital rights, capability expansion, inclusive innovation, and systemic participation. Empirical evidence from developing economies demonstrates that digital inclusion accelerates economic growth, reduces inequality, and strengthens institutional governance. The paper concludes with policy recommendations and future research directions.