<p>The present study introduces the concept of online argumentative discourse to address a limitation in much existing social media discourse research, which often struggles to capture the interactional and evolving dynamics of online controversy as it unfolds in comment threads. It treats online disputes as a dynamic process rather than a static set of texts, in which participants publicly negotiate credibility, evidence, and norms of judgement. Empirically, the article examines the Jiang Ping controversy (in Chinese: 姜萍事件), a heated online debate in China that emerged after a 17-year-old vocational student’s unexpectedly strong performance in a major mathematics competition sparked both widespread admiration and subsequent public skepticism regarding its legitimacy. Using a framework of legitimation and delegitimation within Wodak’s discourse historical approach, the study conducts a thematic analysis of 559 Weibo comments to map how different positions are formed and contested. The analysis identifies three broad commenter categories, sceptics, supporters, and neutral observers, and shows how their argumentative strategies mobilise broader tensions around gender, class, educational hierarchy, and media responsibility, while raising practical concerns about privacy protection, educational elitism, and gender equality.</p>