Background: Pregnancy is a high-risk time for excessive weight gain. The rising prevalence of obesity in women, combined with excess weight gain during pregnancy, means that there are more women with obesity in the postnatal period. This can have adverse health consequences for women in later life and increases the health risks during subsequent pregnancies. Objective: The primary aim was to produce evidence of whether or not a Phase III trial of a brief weight management intervention, in which postnatal women are encouraged by practice nurses as part of the national child immunisation programme to self-monitor their weight and use an online weight management programme, is feasible and acceptable. Design: The research involved a cluster randomised controlled feasibility trial and two semistructured interview studies with intervention participants and practice nurses who delivered the intervention. Trial data were collected at baseline and 3 months later. The interview studies took place after trial follow-up. Setting: The trial took place in Birmingham, UK. Participants: Twenty-eight postnatal women who were overweight/obese were recruited via Birmingham Women’s Hospital or general practices. Nine intervention participants and seven nurses were interviewed. Interventions: The intervention was delivered in the context of the national child immunisation programme. The intervention group were offered brief support that encouraged self-management of weight when they attended their practice to have their child immunised at 2, 3 and 4 months of age. The intervention involved the provision of motivation and support by nurses to encourage participants to make healthier lifestyle choices through self-monitoring of weight and signposting to an online weight management programme. The role of the nurse was to provide regular external accountability for weight loss. Women were asked to weigh themselves weekly and record this on a record card in their child’s health record (‘red book’) or using the online programme. The behavioural goal was for women to lose 0.5-1 kg per week. The usual-care group received a healthy lifestyle leaflet. Main outcome measures: The primary outcome was the feasibility of a Phase III trial to test the effectiveness of the intervention, as assessed against three traffic-light stop-go criteria (recruitment, adherence to regular self-weighing and registration with an online weight management programme). Results: The traffic-light criteria results were red for recruitment (28/80, 35% of target), amber for registration with the online weight loss programme (9/16, 56%) and green for adherence to weekly self-weighing (10/16, 63%). Nurses delivered the intervention with high fidelity. In the qualitative studies, participants indicated that the intervention was acceptable to them and they welcomed receiving support to lose weight at their child immunisation appointments. Although nurses raised some caveats to implementation, they felt that the intervention was easy to deliver and that it would motivate postnatal women to lose weight. Limitations: Fewer participants were recruited than planned. Conclusions: Although women and practice nurses responded well to the intervention and adherence to self-weighing was high, recruitment was challenging and there is scope to improve engagement with the intervention.