Background: Adolescent girls in Nigeria face multiple barriers to physical activity (PA), yet qualitative evidence on their lived experiences remains limited. Understanding these experiences is essential for developing culturally appropriate interventions to promote active lifestyles. This study explored the nature and patterns of PA participation, the enablers and barriers influencing engagement, and strategies for improving PA among adolescent girls in Enugu State, Nigeria. Methods: A qualitative descriptive study was conducted among 20 adolescent girls aged 10–19 years using semi-structured interviews. Data were analysed using reflexive thematic analysis, guided by the Socio-Ecological Model (SEM). The study was reported in accordance with the Consolidated Criteria for Reporting Qualitative Research (COREQ). Results: Four themes were identified: (1) PA was predominantly unstructured and embedded within domestic chores and routine walking rather than intentional exercise; (2) participation was facilitated by parental support, teacher encouragement, peer influence, and access to school facilities; (3) barriers included academic demands, restrictive gender norms, school hierarchy, psychological factors, physical discomfort, and environmental constraints; and (4) participants recommended structured school-based programmes, equitable opportunities, improved facilities, and health education to promote PA. Conclusions: Physical activity participation among adolescent girls was shaped by interacting individual, interpersonal, organisational, and environmental factors. Beyond commonly reported barriers, this study identifies school hierarchy as an important organisational influence that may restrict opportunities for physical activity participation. These findings highlight the need for multilevel, gender-sensitive interventions that strengthen supportive school environments, address sociocultural and organisational barriers, and improve equitable access to safe opportunities for regular physical activity.