This study determined the role of the portfolio assessment as a learner-centered assessment in enhancing learners’ grammatical competencies of students in writing during the third quarter of School Year 2023-2024. The researcher used descriptive rubrics to determine the gain scores in writing through portfolio assessment. This study utilized quantitative research design that draws statistical conclusion from quantitative data. The students' performances in writing have been described in terms of portfolio assessment highlighting the grammatical competencies of the students in terms of Lexis and Spelling, Grammar and Punctuation, Cohesion and Coherence. As a result from the statistical analysis, there is a significant difference in the mean pre-writing and post-writing test scores of the students in the portfolio-assessed reflective learning activities across all three grammatical competencies although the study concludes Grade 8 students need to further enhance their level of grammatical competence in cohesion and coherence compared to lexis and spelling and grammar and vocabulary. The merits and essentials of the findings were drawn and treated as inputs in the development of portfolio assessment in enhancing the grammatical competencies in writing. Recommendations are offered to improve strategies and techniques in instruction and in the development of this strategy will complement the goal to develop the relevant competencies of students in the field of English language learning.