Many physical activity (PA) interventions implemented to tackle the child obesity epidemic have shown limited effectiveness, possibly due to a lack of consideration of potential stress that accompanies behavior adaptation and the automatic perseverative cognition that exacerbates the stress (namely rumination). Purpose: The main aim of this paper is to develop and validate the PA-specific Rumination Scale for Children (PARSC) that assesses children’s tendencies to engage in repeated negative thoughts about PA (Study 2). Items in the scale were derived from qualitative information about factors that inherently demotivates PA participation (intrinsic barriers) through the lived experience of UK children (Study 1). Methods: For Study 1, pedometry PA data were collected from 143 children (aged 6-10 years). Twenty-one focus groups were formed based on participants’ year group, sex and PA level. For PARSC validation (Study 2), 382 children completed the questionnaire twice. Self-reported PA, objective PA and avoidant coping were also assessed. Results: Study 1 – Four overarching themes identified as intrinsic barriers were lack of competence, fear of negative experiences, external constraints and lacking a sense of purpose. Altogether, ten higher order and lower order themes were used to construct PARSC items. Study 2 - From Rasch analysis, PARSC possessed sound internal validity, internal consistency and test-retest reliability. Self-reported PA and avoidant coping were predictive of PA-specific rumination, but not objective PA. Conclusion: PARSC is a useful tool to identify children ruminative about PA for whom interventions can be designed, with the intrinsic barriers considered, to promote PA behavior adaptation.