Background: To investigate how a behavioural lifestyle intervention influences psychosocial determinants of physical activity and dietary behaviours in a population at risk of type 2 diabetes (T2DM). Methods: Fifty-nine women with a body mass index of ≥25 kg/m2 and a history of gestational diabetes mellitus (GDM) participated in a randomized controlled study. The intervention group (n = 29) received 2 face-to-face and 5 telephone lifestyle-counselling sessions with a health professional. The control group (n = 30) received care as usual. At baseline and 6 months, psychosocial determinants related to physical activity and diet were measured with a self-administrated questionnaire. Linear regression analyses were applied to test for intervention effects. Results: The intervention was effective in improving social support (β = 3.5, P < 0.001; β = 2.1, P = 0.02), modifying self-efficacy (β = -2.2, P = 0.02; β = -4.3, P < 0.001), and reducing barriers (β = -3.5, P = 0.01; β = -3.8, P = 0.01) for, respectively, physical activity and diet from baseline to 6-month follow-up in the intervention group compared with the control group. The intervention reduced the following barriers to a physically active lifestyle: lack of energy and lack of motivation. Physical activity barriers like lack of time and lack of childcare were unchanged. The intervention reduced the following barriers to a healthy diet: lack of time, costs, having unhealthy snacks at home, and having cravings for sweets. Conclusion: This lifestyle intervention influenced psychosocial determinants relevant for overweight women with a history of gestational diabetes mellitus (GDM) in prevention of T2DM.