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Abstract Background Engaging in physical activity (PA) during adolescence is beneficial for health and positive development. However, most adolescent girls have low PA levels, and there is a need for interventions outside of school hours. This pilot randomised controlled trial aimed to explore the preliminary effectiveness of three different remote PA interventions in increasing adolescent girls’ moderate-to- vigorous PA (MVPA), fitness and psychosocial outcomes. Methods Girls living in the UK or Ireland, aged between 13 and 16 years old, who wished to increase their activity levels, were eligible for the study. Using a random number generator, participants (n = 153; 14.8y ± 1.4) were randomised into one of three 12-week intervention groups (i) PA programme, (ii) Behaviour change support, or (iii) Combined PA programme and Behaviour change support, or (iv) a Comparison group. Outcome measures included accelerometer and self-reported PA, physical fitness (cardiorespiratory fitness; 20 m shuttle run, muscular endurance; push up, muscular strength; long jump), and psychosocial assessments (perceived competence; body appreciation; self-esteem; behavioural regulation). Linear mixed models were used to analyse differences between each intervention arm and the comparison group immediately postintervention (12 weeks) and at follow up (3-months post-intervention), while adjusting for potential confounders. Results Participation in the PA programme group was associated with higher perceived competence (0.6, 95% CI 0.1 to 1.2), identified regulation (0.7, 95% CI 0.2 to 1.1) and intrinsic motivation (0.9, 95% CI 0.2 to 1.6) at post-intervention. Participation in the Behaviour change group was associated with higher perceived competence at post-intervention (0.6, 95% CI 0.1 to 1.2), and higher push-up scores at the 3-month follow-up (4.0, 95% CI 0.0 to 7.0). Participation in the Combined group was also associated with higher perceived competence at post-intervention (0.8, 95% CI 0.2 to 1.4), and higher push-up scores at the 3-month follow-up (5.0, 95% CI 1.0 to 8.0). No other significant differences were found between the intervention arms and the comparison group. Conclusion Results suggest perceived competence increased across all intervention arms, while the PA programme group enhanced autonomous motivation in the short term. Intervention arms with behaviour change support appear most promising in improving muscular endurance. However, a larger scale trial is needed for a better understanding of between-group differences and the impact of intervention arms on MVPA and fitness, given the small sample size and short-term follow-up.