BackgroundSocio-behavioural adaptations during the COVID-19 pandemic may have significantly affected adolescents’ lifestyle. This study aimed to explore possible reasons affecting changes in physical activity and sedentary behaviour in Indonesian adolescents during the pandemic based on mothers’ perspectives.MethodsWe recruited parents (n = 20) from the Yogyakarta region of Indonesia (July–August 2020) using purposive and snowball sampling. Individual interviews were audio-recorded, transcribed verbatim and anonymised. Data were imported into NVivo software for a reflexive thematic analysis.ResultsThe interviews lasted between 38 and 113 min (M = 65 min). Participants’ age ranged between 36 and 54 years (M = 42.6 years). Participants’ children ranged in age from 12 to 15 years (M = 13.7 years, female: 9, male: 11). Themes related to changes in physical activity during the pandemic were 1) self-determination and enjoyment, 2) supports from others, and 3) physical activity facilities and equipment. Themes related to changes in sedentary behaviour during the pandemic included 1) educational demands, 2) psychological effects due to the pandemic, 3) devices and internet availability, 4) parental control, and 5) social facilitators.ConclusionsDuring the pandemic, mothers perceived their children to be less active and using more screen-based devices, either for educational or recreational purposes, compared to before. The present themes might be useful when developing interventions and policies promoting physical activity and reducing sedentary behaviour in adolescents. Interventions could, for example, consider increasing parents’ and adolescents’ awareness on current activity guidelines, providing education on healthier recreational screen time, and involving parents, peers, and teachers. Increasing the accessibility of physical activity facilities and equipment, making use of adolescents’ favourite program and social media for interventions, and providing activities that are fun and enjoyable may also important.