This study examines the relationships between grammar proficiency, self-efficacy, and writing outcomes among Senior High School students at San Antonio National High School. The research addresses the critical problem of students’ difficulty in producing coherent, grammatically accurate, and well-structured written work, a challenge compounded by low self-efficacy. Using a correlational research design, data were collected from 102 students via validated survey questionnaires. The study measured grammar proficiency (focusing on error detection and tense consistency), self-efficacy (confidence and resilience), and writing outcomes (clarity, coherence, grammar, and mechanics). Key findings reveal a moderately high level of grammar proficiency and high self-efficacy among students, with significant positive correlations between grammar proficiency and writing outcomes (r = 0.852, p < 0.000), and self-efficacy and writing outcomes (r = 0.798, p < 0.000). Grammar proficiency also strongly correlates with self-efficacy (r = 0.852, p < 0.000), indicating its foundational role in fostering confidence and resilience in writing tasks. These findings underscore the interplay between cognitive (grammar skills) and affective (self-efficacy) factors in shaping writing performance. The study highlights the need for explicit grammar instruction, self-efficacy-building strategies, and targeted interventions to enhance students' writing skills. Implications suggest that curriculum designers, educators, and policymakers should prioritize integrated approaches that address both grammar and self-belief to improve academic writing outcomes.