Background: Physical activity is reduced during the post-partum period. Facebook is frequently used by Australian mothers, and offers flexibility, high levels of engagement and the ability to disseminate information and advice via social contacts. The Mums Step it Up Program is a newly developed 50 day team-based physical activity intervention delivered via a Facebook app. The program involves post-partum women working in teams of 4-8 friends aiming to achieve 10,000 steps per day measured by a pedometer. Women are encouraged to use the app to log their daily steps and undertake social and supportive interactions with their friends and other participants. This study aims to determine the effectiveness of the Mums Step it Up Program. Method/design. A sample of 126 women up to 12 months post-partum will be recruited through community-based health and family services. Participants will be randomly allocated into one of three groups: control, pedometer only and the Mums Step it Up Program. Assessments will be completed at baseline, 6 weeks and 6 months. The primary outcome (objective physical activity) and the secondary outcomes (sleep quality and quantity, depressive symptoms, weight and quality of life) will be used to determine the effectiveness of the Mums Step it Up Program compared with the control and pedometer only groups. Analyses will be undertaken on an intention-to-treat-basis using random effects mixed modeling. The effect of theorized mediators (physical activity attitudes, subjective norms and perceived behavioral control) will also be examined. Discussion. This study will provide information about the potential of a Facebook app for the delivery of health behavior interventions. If this intervention proves to be effective it will be released on a mass scale and promoted to the general public. Trial registration. Australia and New Zealand Clinical Trials Register: ACTRN12613000069752.