Background: Young children are prone to low levels of physical activity in childcare. This environment, inclusive of equipment, policies, and staff, has been identified as influencing young children’s activity behaviours. To date, no study has examined the feasibility and effectiveness of such policies in Canadian childcare centres, while the provision of physical activity policies in other countries has shown some promise for improving the activity levels of young children. As such, the primary objective of the Childcare PhysicaL ActivitY (PLAY) Policy study is to examine the feasibility of an evidence-based, stakeholder-informed, written physical activity and sedentary time policy for centre-based childcare (i.e., at the institutional level). The secondary objectives are to examine the impact of policy implementation on the physical activity levels and sedentary time of young children, subsequent environmental changes in childcare centres, and childcare providers’ self-efficacy to implement a physical activity policy. This study will examine both policy implementation and individual (behavioural) outcomes. Methods/Design: The Childcare PLAY Policy study, a pilot, cluster-randomized controlled trial, involves the random allocation of childcare centres to either the experimental (n = 4) or control (n = 4) group. Childcare centres in the experimental group will adopt a written physical activity policy for eight weeks (at which time they will be asked to stop enforcing the policy). Physical activity levels and sedentary time in childcare will be assessed via ActiGraph™ accelerometers with measurements at baseline (i.e., week 0), mid-intervention (i.e., week 4), immediately post-intervention (i.e., week 9), and at six-month follow-up. Policy implementation and feasibility will be assessed using surveys and interviews with childcare staff. The Environment and Policy Assessment and Observation Self-Report tool will capture potential changes to the childcare setting. Finally, childcare providers’ self-efficacy will be captured via a study-specific questionnaire. A nested evaluation of the impact of policy implementation on young children’s physical activity levels will be completed. A linear mixed effects models will be used to assess intervention effects on the primary and secondary outcomes. Descriptive statistics and thematic analysis will be employed to assess the feasibility of policy implementation. Discussion: The Childcare PLAY Policy study aims to address the low levels of physical activity and high sedentary time observed in childcare centres by providing direction to childcare staff via a written set of evidence-informed standards to encourage young children’s activity and reduce sedentary time. The findings of this work will highlight specific aspects of the policy that worked and will inform modifications that may be needed to enhance scalability. Policy-based approaches to increasing physical activity affordances in childcare may inform future regulations and programming within this environment.