This paper explores the emerging concept of Cognitive Sovereignty — the right and ability of individuals and societies to maintain autonomous control over mental processes such as attention, perception, and decision-making in an age dominated by digital platforms and algorithmic influence. It argues that modern technologies, through data-driven design and behavioural manipulation, pose profound challenges to human autonomy, democratic discourse, and cultural evolution. The study outlines how algorithmic systems, optimised for engagement rather than truth, reshape cognition and fragment shared reality. It proposes a multi-level framework for reclaiming cognitive sovereignty through media literacy, institutional reform, regulatory oversight, ethical design, and technical interventions such as open-source algorithms and decentralised platforms. Ultimately, the paper positions cognitive sovereignty as a national and human security imperative, asserting that safeguarding independent thought and decision-making is central to democracy, dignity, and the future of human agency in the digital age.