Energy security has become a central concern in contemporary energy policy, particularly amid the global transition toward sustainable energy systems and intensifying geopolitical uncertainty. Traditionally conceptualized as the uninterrupted availability of energy resources at affordable prices, the notion has evolved into a multidimensional framework encompassing environmental sustainability, technological innovation, and complex geopolitical dynamics. Recent events, including the global energy crisis of 2021–2022 and the disruption of European gas supplies due to the Russia–Ukraine war, have exposed structural vulnerabilities in energy markets and reinforced the necessity for resilient, diversified, and strategically adaptive energy systems. This thesis investigates sustainable energy security through a comparative and multidimensional analytical framework integrating economic, environmental, and geopolitical dimensions. The primary objective is to critically assess how selected countries navigate the intersection of energy supply reliability, sustainability imperatives, and geopolitical risks within their national energy strategies. Methodologically, the study combines a thorough review of scholarly literature with a comparative assessment using a set of quantitative and qualitative energy security indicators. These indicators allow for a systematic evaluation of national energy system performance, highlighting relative strengths, weaknesses, and resilience against global energy disruptions. The approach emphasizes not only technical and economic dimensions but also strategic decision-making and policy alignment with long-term sustainability goals. Empirical analysis focuses on nine major economies: the United States, Canada, the United Kingdom, Germany, France, Japan, India, China, and Australia. By examining both indicator-level performance and the design of national energy policies, the research identifies recurring patterns, strategic priorities, and structural vulnerabilities. The findings offer critical insights into the formulation of robust energy policies and contribute to advancing sustainable energy security frameworks capable of addressing the increasingly complex and uncertain global energy landscape.