There is a paucity of research pertaining to the discourse-based construction of legitimation within the realms of law and healthcare. To fill this gap, the present study examines the deliberations within the Legislative Council of Hong Kong pertaining to the enactment of legislation recognizing rare diseases (RDs) as eligible for governmental support. Employing van Leeuwen’s sociosemantic approach, we analyzed the debates, revealing four distinct discursive strategies, namely rationalization, morality, authorization, and mythopoesis, employed by the legislators to advocate for RD legislation. Among these strategies, rationalization emerged as the most frequently employed. The legislators’ discourse carries ideological implications concerning humanitarianism. This study not only sheds light on the utilization of legitimation strategies to advocate for the enactment of social policies within a legal context but also provides valuable insights for other regions seeking to advance RD legislation from a linguistic perspective.