Background: Adolescent females physical activity (PA) participation rates are low globally, particularly among females of lower socio-economic status (SES). Evidence suggests theory‐based, multi-component interventions are most effective at improving PA levels. Following the Medical Research Council framework, this research aimed to co-design, with adolescent females, a theory-driven, multi-component, extra-curricular school-based PA intervention, the Girls Active Project (GAP), and assess its feasibility. Setting: One single-sex, female-only, designated disadvantaged post-primary school in Dublin, Ireland. Methods: The Behaviour Change Wheel (BCW) and Public and Patient Involvement (PPI) were used to develop the GAP programme. Mixed-methods with students (n=287, aged 12-18) and teachers (n=7) captured students’ self-reported PA levels and identified factors influencing PA behaviour at school. These data were subsequently used in discussion groups with PPI contributors (n=8, students aged 15-17) to co-design the intervention. Mixed-methods were applied with multiple stakeholders to assess the feasibility of implementing and evaluating the GAP programme over a 12-week single-arm feasibility trial. Results Just 1.4% of the students in this sample (n=287, aged 12-18) reported meeting the recommended PA guidelines. Time, social influences, beliefs about capabilities, environmental context and resources, goals, reinforcement, and behavioural regulation emerged from the data as factors influencing PA behaviour. A peer-led, after-school PA programme was co-designed. The feasibility study encountered significant contextual barriers and challenges with recruitment. Recruitment (n=8, 10%) was low, yet retention (n=7, 88%) was high. Despite the COVID-19 pandemic hindering implementation, results suggested the GAP programme was implemented with high fidelity (87%), well-received by stakeholders and perceived as compatible with the after school-setting. Conclusions: PA levels of females in this sample were far below recommended guidelines for optimum health. The novel approach applied to systematically co-design the intervention could facilitate future replication. Whilst further thought must be given on how to increase enrolment, the in-person delivered PA programme showed promise as an intervention that can be feasibly implemented and evaluated. Future research should examine the GAP’s preliminary-effectiveness at increasing PA levels in a pilot-cluster randomised controlled trial