This study aimed at exploring the effects of inquiry-based learning on students’ writing performance, critical thinking skills, and motivation towards academic writing. A quasiexperimental design that employed time-series design with single group participants was used. A total of 21 EFL undergraduate students who took advanced writing skills course were selected using comprehensive sampling method. Tests, focus group discussion, and student-reflective journal were used to gather data. The participants were given three argumentative essay writing pre-tests and other three argumentative essay writing post-tests before and after the intervention, inquiry-based argumentative essay writing instruction. While the quantitative data were analysed using one-way repeated measures MANOVA, paired-samples t-test, and descriptive statistics, the qualitative data were analysed through narration. The findings revealed that using inquiry-based learning enhanced students’ writing performance, critical thinking skills, and increased students’ motivation towards writing. This method developed students’ writing performance in terms of task achievement, coherence and cohesion, lexical resource, and grammatical range and accuracy. Besides, it enhanced students' interpretation, analysis, evaluation, inference, explanation, and self-regulation skills which are the core critical thinking skills. It also increased students’ motivation towards writing which includes self-efficacy, goal orientation, belief and affect about writing. Therefore, inquiry-based learning is suggested as a means to improve students’ writing performance, critical thinking skills, and increase students’ motivation towards writing. Thus, this study recommends researchers, teachers, and students to pay due attention to inquiry-based learning in their academic journey. Keywords: Inquiry-based learning, Writing performance, Critical thinking skills, motivation