This study examined the relationships between social support from parents, friends, and teachers, grit, and academic achievement among junior high school students in South Korea. Data were collected from 9502 students using third-grade junior high school students in the 2020 National Level Assessment of Academic Achievement. Descriptive statistics, regression, and mediator effect analysis were performed by using SPSS 21.0 and Hayes Process Macro 3.5. The results indicated that parental support had a greater impact on learners' academic achievement than support from teachers or peers. In addition, grit mediated the relationship between social support and learners' academic achievement, and teachers had a larger effect on learners' grit than support from parents or peers. These findings suggest that schools and educational institutions should consider incorporating parental involvement in programs to enhance academic achievement. In addition, the positive impact of social support from teachers, parents, and friends on grit highlights the benefits of implementing grit development programs within educational settings.