Background Weight gain is commonly observed during and after breast cancer treatment and is associated with poorer survival outcomes, notably in women with oestrogen-receptor positive disease. The aim of this qualitative study was to investigate the experiences and perceptions of oestrogen-receptor positive (ER +) female breast cancer patients (BCPs) regarding weight management behaviours during and after treatment. Secondly, to gain insight into the experiences of healthcare professionals (HCPs) regarding the provision of weight management advice to patients undergoing treatment. Methods Four focus groups involving 16 BCPs having a median (range) age of 51 (35–70 y) and three focus groups involving 21 HCPs aged 46 (29–62) were held at a university campus, local cancer support centre or clinical site. Data were analysed using Framework analysis. Results Four overarching themes (and 10 subthemes) were identified: (1) Treatment; (2) Support for lifestyle behaviour change; (3) Information availability for BCPs; (4) Knowledge of current evidence amongst HCPs. The physical and psychological consequences of treatment influenced motivation for weight management amongst BCPs. Social support for health promoting behaviours was viewed as important but was conflicting, requiring context-specific considerations. BCPs said they would have welcomed access to credible information (guided by HCPs) about the potential detrimental health effects of excess body weight and weight gain, together with advice on weight management via healthy eating and physical activity. HCPs felt that they had insufficient knowledge of public health dietary and physical activity recommendations or evidence-based interventions to confidently offer such advice. HCPs expressed concern that raising weight management issues would exacerbate distress or invoke feelings of guilt amongst BCPs, and cited time pressures on patient consultations as additional barriers to providing weight management support. Conclusion The study yielded novel insights into factors influencing weight management behaviours amongst overweight ER + BCPs. The results suggest that evidence-based information and support, which addresses key physical and psychological challenges to physical activity and dietary behaviours, offers the best route to sustainable weight management in this population.