Purpose: Women who are diagnosed with gestational diabetes mellitus have an increased risk of developing type 2 diabetes, but most receive little guidance regarding disease prevention. This study examined the role and usefulness of social support, including peer counselling in facilitating behaviour change as a part of a healthy eating and physical activity intervention among women with a previous gestational diabetes mellitus pregnancy. Methods: We used a qualitative descriptive approach to investigate participants' experiences with the social support they received during the intervention. We used purposeful sampling and invited women and peer counsellors to participate in semi-structured interviews. Data were analyzed using content analysis. Results: We interviewed nine women and two peer counsellors. Participants received emotional, appraisal, and informational types of social support from the peer counsellor and exercise specialist that they reported as useful. Additionally, participants' received informal emotional and instrumental support from family, friends, and colleagues that they found useful in supporting behaviour change. Conclusions: Different types of social support are important to encourage behaviour change. These findings provide insight into the types of social support women with previous gestational diabetes mellitus find useful, in addition to practical ways that social support can be incorporated into future interventions.