Background Girls are less active than boys and few adolescent girls meet physical activity (PA) guidelines. Peers are an important influence on the views and behaviours of adolescent girls, yet many PA interventions involving peers use formal approaches that may not harness the power of peer groups. More informal peer-led PA interventions, which work within proximal peer groups, may hold promise for increasing girls’ PA. Objectives To examine the feasibility, evidence of promise and cost of the Peer-Led physical Activity iNtervention for Adolescent girls (PLAN-A), a peer-led PA intervention. Design Phase 1 comprised formative work and a pilot study conducted in one secondary school. Phase 2 was a feasibility study comprising a pilot randomised controlled trial in six secondary schools, including process and economic evaluations. Setting Six secondary schools in South Gloucestershire and Wiltshire, recruited from schools above the median local Pupil Premium (i.e. more deprived). Participants Year 8 girls (aged 12–13 years). Intervention Year 8 girls nominated other girls in their year who are likely to be influential (e.g. who they look up to, are good listeners); the 18% most nominated were invited to be peer supporters (PSs). PSs attended 2 consecutive days of training (plus a top-up day 5 weeks later) outside the school site, led by pairs of PS trainers, to increase their knowledge about PA and their capabilities and confidence to promote PA in their friendship group. Main outcome measures Measures focused on establishing evidence for feasibility and promise: recruitment and retention of Year 8 girls and PSs, data provision rates [accelerometer and questionnaire collected pre randomisation/beginning of Year 8 (T0), end of Year 8 (T1) and beginning of Year 9 (T2)], intervention acceptability, PS training attendance, intervention cost, and the between-arm difference in weekday minutes of moderate to vigorous PA (MVPA). A process evaluation was conducted. Results Six schools were recruited: four PLAN-A (n = 269) and two control (n = 158). In total, 94.7% of Year 8 girls participated. A total of 55 (17–24% of Year 8 girls) PSs were trained (attendance rate 91–100%). Five girls were trained as PS trainers. Questionnaire data provision exceeded 92% at all time points. Accelerometer return rates were > 85% and wear-time criteria were met by 83%, 71% and 62% of participants at T0, T1 and T2, respectively. Mean weekday MVPA did not differ between intervention arms at T1 (1.1 minutes, 95% CI –4.3 to 6.5 minutes) but did at T2 (6.1 minutes, 95% CI 1.4 to 10.8 minutes), favouring PLAN-A. The mean cost of intervention delivery was £2685 per school or £37 per Year 8 girl. Process evaluation identified good fidelity, engagement and enjoyment of the PS training and peer-support strategies. PSs needed more guidance on how to start conversations. Limitations Accelerometer data provision was lowest at T2, suggesting a need for strategies to increase compliance. Conclusions Informal peer-led intervention approaches, such as PLAN-A, hold promise as a means of promoting PA to adolescent girls. Future work A definitive randomised controlled trial of PLAN-A is warranted. Trial registration Current Controlled Trials ISRCTN12543546. Funding This project was funded by the National Institute for Health Research (NIHR) Public Health Research programme and will be published in full in Public Health Research; Vol. 7, No. 16. See the NIHR Journals Library website for further project information. The work was undertaken with the support of the Centre for the Development and Evaluation of Complex Interventions for Public Health Improvement (DECIPHer), a UK Clinical Research Collaboration (UKCRC) Public Health Research Centre of Excellence. Joint funding (MR/KO232331/1) from the British Heart Foundation, Cancer Research UK, Economic and Social Research Council, Medical Research Council, the Welsh Government and the Wellcome Trust, under the auspices of the UKCRC, is gratefully acknowledged. This study was designed and delivered in collaboration with the Bristol Randomised Trials Collaboration (BRTC), a UK CRC-registered clinical trials unit in receipt of NIHR clinical trials unit support funding. The intervention costs were jointly funded by South Gloucestershire Council and Wiltshire Council.