Background: To maintain increases in physical activity (PA), interventions that implement group dynamics principles and strategies with the intent of enhancing group cohesion may be advantageous. This study examined group cohesion and PA following a group dynamics-based PA intervention among breast cancer survivors. Methods: The study was designed as a pilot randomized controlled trial comparing an 8-week group dynamics-based intervention with an individually supervised intervention. Group cohesion was measured by the Physical Activity Group Environment Questionnaire, and PA was measured at baseline, post-intervention, and 3-month follow-up using a self-report questionnaire and pedometer. Results: Group cohesion levels were high following the intervention and positively associated with PA at 3-month follow-up (ranger = .182–.555). At 3-month follow-up, 91.7% of participants in the group-dynamics-based intervention (n = 12) were classified as moderately active or greater, compared with 54.5% in the individually supervised intervention (n = 11). Conclusions: These results suggest that, for breast cancer survivors, peer support and fostering group cohesion as part of an exercise program may help to support PA following the completion of a structured intervention. A larger trial with longer follow-up is needed to establish comparative efficacy for a group-dynamics-based exercise intervention to enhance long-term PA adherence in breast cancer survivors.