There is a lack of research on girls' physical activity (PA) in secondary schools in East Asian contexts, and in South Korea in particular, where the PA rates of adolescents are considerably low, especially for females. The purpose of this study was to investigate South Korean physical education (PE) teachers' and female secondary school students' beliefs about what influences girls' PA participation with a particular focus on relational factors. Employing the theory of planned behavior (TpB) and a qualitative research design, data were collected from six PE teachers and 12 female students in public middle schools and high schools in South Korea through demographic questionnaires, a series of semi-structured interviews with teachers and students, and non-participant observations of the PE teachers' classes. Using thematic analysis with TpB to guide the coding, findings indicated that relationships with classmates and teachers influenced girls' PA engagement in PE in complex ways, yielding meaningful pedagogical implications.