Abstract This study examined the effects of problem-based English writing instruction (PBEWI) on university students’ critical thinking dispositions, writing self-efficacy, and argumentative writing skills. To this effect, between groups quasi-experimental design was employed through which data were collected from control and experimental groups using questionnaires, tests, learning journals, and textual analysis. Based on this, the data were analyzed using One-way MANOVA and thematic qualitative analysis. The findings of the overall tests showed that the instruction brought positive general effects on the experimental group students’ critical thinking dispositions, writing self-efficacy, and argumentative writing skills. However, the results of the separate tests on three dependent variables indicated that there were significance differences. In other words, the experimental group showed positive significant changes in critical thinking dispositions and argumentative writing skills. In contrast, there was no significant change regarding writing self-efficacy although students in the experimental group showed progress in the descriptive statistics. As to the qualitative findings, students reflected in their learning journals that PBEWI improved their critical thinking dispositions, writing self-efficacy, and argumentative writing skills. Correspondingly, the textual analysis of the students’ writing clearly revealed that the experimental group produced better argumentative writing than that of the control group. Based on the findings of the study, it was concluded that PBEWI helped students improve their critical thinking dispositions, writing self-efficacy, and argumentative writing skills. Finally, it was recommended that: 1) PBEWI should be incorporated in the course English for Lawyers to improve Law students’ writing skills; 2) instructors should use authentic materials that are suitable, relevant and interesting for classroom discussions to enable students to be more critical in articulation, reason and argumentation while teaching writing skills.