OBJECTIVES: To develop a core outcome set (COS) for physical activity interventions in primary schools. DESIGN: Modified-Delphi study. SETTING: The UK and international. PARTICIPANTS: 104 participants from four stakeholder groups (educators, public health professionals, health researchers, parents); 16 children (aged 8-9 years) from 1 London primary school. INTERVENTIONS: Physical activity interventions. METHODS: Four-stage process: (1) outcomes extracted from relevant studies identified from an umbrella review and a focus group; (2) list of outcomes produced and domains established; (3) stakeholders completed a two-round Delphi survey by rating (Round 1) and re-rating (Round 2) each outcome on a nine-point Likert Scale from 'not important' to 'critical': a>70% participant threshold identified the outcomes rated 'critical' to measure, and outcomes important to children were identified through a workshop; and (4) a stakeholder meeting to achieve consensus of the outcomes to include in the COS. RESULTS: In total, 74 studies were extracted from 53 reviews. A list of 50 outcomes was produced and three domains were established: 'physical activity and health' (16 outcomes), 'social and emotional health' (22 outcomes) and 'educational performance' (12 outcomes). 104 participants completed survey Round 1; 65 participants completed both rounds. In total, 13 outcomes met the threshold; children identified 8 outcomes. Fourteen outcomes achieved consensus to produce the COS: five outcomes for physical activity and health (diet (varied and balanced), energy, fitness, intensity of physical activity, sleep (number of hours)); seven outcomes for social and emotional health (anxiety, depression, enjoyment, happiness, self-esteem, stress, well-being); and two outcomes for educational performance (concentration, focus). CONCLUSIONS: We have developed the first COS for physical activity interventions in primary schools in consultation with those interested in the development and application of an agreed standardised set of outcomes. Future studies including these outcomes will reduce heterogeneity across studies. TRIAL REGISTRATION NUMBER: Core Outcome Measures in Effectiveness Trials Initiative registration number 1322; Results.