This article reports the findings of an action research which aimed at describing and analyzing the impact that the implementation of argumentative writing activities through a critical literacy approach may have on 4th semester pre-service teachers’ argumentative competence development. Participants belong to the B.A. program in English and Spanish teaching at Universidad Pedagógica Nacional. The article draws on qualitative data collection techniques such as questionnaires, focus groups, audio recordings of class sessions, and class artifacts. Findings indicate that adopting a critical literacy approach helped pre-service-teachers foster the development of their written argumentative competence. It was revealed that participants grew as argumentative writers as they were able to take a clear position and weigh their claims. Moreover, they were engaged in reflection that fostered awareness of their roles as future ELT educators which in turn empowered them to discover teaching as a tool for transformation.