<p>Argumentative writing is a critical indicator of English as a Foreign Language (EFL) writing proficiency. Nevertheless, EFL learners often encounter difficulties in producing effective argumentative essays. Providing students with detailed diagnostic feedback through a structured checklist may offer a useful means of improving their argumentation skills. Accordingly, this study aims to develop and validate a diagnostic checklist specifically designed to assess argumentation in the argumentative writing of EFL undergraduates. The checklist was developed on the basis of Toulmin’s Argument Model and the existing literature on argumentative writing. It was subsequently refined through feedback from six EFL experts and three raters, together with an analysis of student essay samples, resulting in a final 5-point checklist consisting of 14 items with four dimensions. Each item was formulated as a single-sentence descriptor targeting a distinct aspect of argumentation competence. The study employed Partial Least Squares Structural Equation Modeling (PLS-SEM), Many-Facet Rasch Model (MFRM), and Pearson correlation analysis to examine the checklist’s reliability, structural validity, rater severity and consistency, and criterion-related validity. Additionally, qualitative analysis of student perceptions further support for the checklist’s effectiveness in providing detailed diagnostic feedback and promote students’ argumentation skill development. The findings underscore the pedagogical potential of the checklist for supporting both EFL learners and instructors in the learning and teaching of argumentative writing, with implications for classroom practice and further research.</p>