This article explores the multidimensional transformation of international security studies in the aftermath of the September 11, 2001 terrorist attacks, which marked one of the most significant ruptures in the early twenty-first century. The post-9/11 period not only redefined the global security agenda but also revealed the limitations of classical theories such as realism and liberalism, which had long equated security with state survival, military power, and institutional cooperation. The emergence of asymmetric and transnational threats - terrorism, radicalization, cyberattacks, biosecurity risks, environmental crises, large-scale migration, and identity-based conflicts - expanded the conceptual boundaries of security and necessitated theoretical innovation. The study analyzes this transformation through a critical engagement with realism, liberalism, constructivism, post-structuralism, and critical security theories, while also incorporating feminist and post-colonial approaches. These perspectives collectively demonstrate that security cannot be reduced to material capacities or interstate power relations but must be understood as a socially constructed and discursively reproduced phenomenon with deep normative implications. The securitization framework of the Copenhagen School provides a useful analytical tool for understanding how political actors framed new threats as existential, thereby legitimizing exceptional measures such as the Patriot Act, Guantanamo practices, and global surveillance regimes. This process simultaneously exposed tensions between security and democracy, freedom and control, as well as national sovereignty and global governance. Methodologically, the article adopts a qualitative and interpretive design, drawing upon conceptual analysis, discourse analysis, and interdisciplinary linkages. Primary documents such as national security strategies, UN Security Council resolutions, and official doctrines are combined with secondary theoretical works to trace how the scope, referent objects, and instruments of security have evolved. Special emphasis is placed on the epistemological pluralism that emerged as boundaries between rationalist (realism, liberalism) and interpretivist (constructivist, post-structuralist, feminist, post-colonial) approaches became more permeable. The findings indicate three key contributions. First, security has undergone a process of conceptual expansion, incorporating environmental, digital, biosecurity, and human security dimensions alongside military concerns. Second, the referent object of security has diversified from the state to individuals, societies, and transnational communities, making security a multi-level and multi-actor phenomenon. Third, the legitimacy of security practices is increasingly shaped by discursive and normative frameworks, highlighting the importance of justice, inclusivity, and human rights. Together, these contributions underscore that the post-9/11 transformation of security cannot be captured by a single theoretical paradigm; rather, it requires hybridization, methodological diversity, and normative sensitivity. Overall, this article argues that the evolution of international security after 9/11 reflects both a paradigm crisis and a conceptual enrichment of the discipline. By bridging classical and critical approaches and integrating interdisciplinary insights from sociology, psychology, and cultural studies, the study provides not only a theoretical synthesis but also practical guidance for policymakers confronting complex and hybrid threats. In this sense, the article contributes to the construction of a more pluralistic, human-centered, and normatively grounded understanding of security in contemporary international relations.