Background <p>Early prevention of Gestational diabetes mellitus (GDM) is critical not only for protecting maternal and infant health but also for reducing the long-term healthcare burden associated with chronic diseases. Physical activity is a well-established modifiable behavior for GDM prevention; however, existing physical activity interventions during pregnancy have shown limited effectiveness. Partner involvement in behaviour change represents a promising approach theoretically, but empirical evidence remains scarce. This study protocol describes a randomized controlled trial designed to evaluate the effectiveness of a couple-based physical activity intervention for the prevention of GDM.</p> Methods <p>This single-blind, two-arm, parallel-group randomized controlled trial will be conducted at an obstetrics and gynecology hospital in Guangzhou, China. A total of 244 pregnant individuals at high risk for GDM and their partners will be randomly assigned to either an intervention group receiving a couple-based physical activity intervention or a control group receiving usual antenatal care. The intervention starts at 5–12^+ 6 weeks of gestation and continues until 24 weeks. It consists of one face-to-face group dyadic education session, one online group dyadic education session, biweekly online personalized dyadic counseling sessions, and a daily behavior change booster delivered via WeChat. Intervention delivery is supported by a dedicated WeChat Official Account, Pregnancy Support Station (孕期加油站), developed by the research team. Primary outcomes are the incidence of GDM, blood glucose levels, and physical activity levels. Secondary outcomes include gestational weight gain, depressive symptoms, potential mediators (physical activity knowledge, fear of physical activity, dyadic coping, and physical activity self-efficacy), and maternal, paternal, and neonatal health outcomes. Data will be collected at 5–12^+ 6, 24–27^+ 6, and 35–36^+ 6 weeks of gestation, and within 3 days after delivery.</p> Discussion <p>This intervention views pregnant individuals and their partners as an interdependent dyad, frames GDM prevention as a shared responsibility within couples, and empowers couples to mobilize interpersonal resources to collaboratively improve maternal physical activity. Findings from the forthcoming randomized controlled trial may help optimize GDM prevention strategies in high-risk populations and support the integration of partner involvement into routine antenatal care.</p> Trial registration <p>Chinese Clinical Trial Registry, ChiCTR2500107402 (registered 11 August 2025).</p>