Despite the critical role of academic English writing proficiency in higher education, existing research has inadequately addressed the comparative impact of teacher-led and student-involved assessments on writing outcomes. To address the research gap, this study aimed to evaluate the effectiveness of 'Teacher-Only Assessment' compared to 'Combined Teacher and Student Self-Assessment' in English writing at a university in southern Thailand. Additionally, the study examined the viewpoints of students regarding the implementation of these approaches. It utilized a sequential explanatory research design in conjunction with a quasi-experimental design. The research instruments included a series of writing tasks, a teacher assessment rubric, a self-assessment form, and a questionnaire with open-ended questions. A group of 62 first-year English majors was divided into two groups: a control group and an experimental group, consisting of 31 students each. Both groups were taking an English Composition I course for 12 weeks and were assigned the same writing tasks. The experimental group was provided with both self-assessments and teacher evaluations, whereas the control group solely received teacher evaluations. The quantitative data was analyzed using an independent sample, while the qualitative data was examined through a thematic analysis. Quantitatively, the independent t-test results indicated that there was no statistically significant distinction in overall writing performance between both groups, except for narrative paragraphs. Moreover, the qualitative findings emphasized the merits of incorporating teacher-led and student-involved assessments. This study suggests various implications for English as a Foreign Language (EFL) instruction and assessment practices.