Contemporary political science faces a growing challenge in explaining why some political systems remain stable despite declining public trust, while others experience institutional crises despite possessing formal democratic structures. Existing theories of legitimacy, institutionalism, social capital, and governance provide important insights but often fail to explain the dynamic interaction between state adaptability and citizen expectations in highly complex societies. This article proposes a new theoretical framework, Adaptive Legitimacy Theory (ALT), which argues that political stability depends not primarily on institutional design or electoral participation alone, but on the capacity of political institutions to continuously adapt to changing societal expectations while maintaining normative legitimacy. The theory introduces the concept of Adaptive Legitimacy Capacity (ALC), defined as the ability of political systems to absorb social, technological, economic, and cultural changes without losing public acceptance. The article develops the theoretical foundations of ALT, identifies its causal mechanisms, derives testable hypotheses, and discusses its implications for democratic governance in the twenty-first century. Keywords: Political Legitimacy, Governance, Institutional Adaptation, Democracy, Political Stability, State Capacity, Public Trust