Higher education provides students with the platform for mobilising knowledge for practical use in the face of unforeseen situations. Referring to the area of media literacy, students nowadays are more likely to have access to a variety of information and publish their ideas; cultivating media literacy quality and skills thus takes on heightened significance. This requires critical thinking which encompasses knowledge and capabilities for achieving understanding, making appropriate judgement, and taking meaningful action, as well as a pedagogical approach to activating learning. The literature suggests that constructivist problembased learning (PBL) has the potential for enhancing critical thinking theoretically; empirically, studies in different disciplines argue for the importance of strategic implementation and supportive facilitation. This study defined critical thinking as a threshold concept and established the epistemological threshold framework with conceptual and practical levels to investigate how PBL contributed to the development of critical thinking in the news media literacy class through students’ learning experiences, academic performance, and perceptions of their development. Thirty-five Taiwanese undergraduates from an Applied English Department in Southern Taiwan participated in this research. Classroom action research was conducted with multiple methods including focus group interviews, questionnaires, and the teacher’s observations, together with assessments of students’ academic group work and individual writing tasks through the PBL process. It was found that the learning journey was explicitly transformative and troublesome, while the integrative, bounded and irreversible characteristics of a threshold concept emerged during the research process. The dynamics of peer and teacher-student collaborative work also suggested students’ and the teacher’s epistemological, practical, and ontological development associated with the cognitive, affective, and social aspects of learning. The data from this study were combined with existing research relating to critical thinking and the pedagogical implications of PBL to develop a reflexive framework for future practice.