<p>This paper examines the compounded threat that <i>post-truth populism</i> poses to the foundations of deliberative democracy. While most scholarship treats populism and post-truth politics as separate phenomena, we argue that their convergence creates a distinct and more corrosive force—one that combines epistemic distortion with political exclusion. By conceptualizing post-truth populism as more than the sum of its parts, we offer new insights into its impact on democratic legitimacy and the conditions for public reason. Making use of Rawlsian terminology and principles, we contend that post-truth populism must be seen as an unreasonable doctrine that compromises the ethical and epistemological basis of deliberative democracy. The paper explores the consequences of this characterization and concludes by examining strategies to counteract these challenges while preserving the core principles of liberal democracy.</p>