PurposeUpdated evidence for the treatment of obesity in cancer survivors includes behavioural lifestyle interventions underpinning at least one theoretical framework. The aim of this systematic review was to assess the effectiveness of theory-based lifestyle interventions for the treatment of overweight/obesity in breast cancer survivors and to report effective behavioural change techniques (BCTs) and components used in these interventions.MethodsFour databases were searched for RCTs published between database inception and July 2022. The search strategy included MeSH terms and text words, using the PICO-framework to guide the eligibility criteria. The PRISMA guidelines were followed. Risk-of-bias, TIDier Checklist for interventions’ content, and the extent of behaviour change theories and techniques application were assessed. To evaluate the effectiveness of interventions, trials were categorised as “very,” “quite,” or “non” promising according to their potential to reduce body weight, and BCTs promise ratios were calculated to assess the potential of BCTs within interventions to decrease body weight.ResultsEleven RCTs met the inclusion criteria. Seven trials were classified as “very”, three as “quite” and one study was “non” promising. Studies’ size, design, and intervention strategies varied greatly, but the weight-loss goal in all studies was ≥ 5% of the initial body weight through a 500–1000 kcal/day energy deficit and a gradually increased exercise goal of ≥ 30 min/day. Social Cognitive Theory was the most commonly used theory (n= 10). BCTs ranged from 10 to 23 in the interventions, but all trials included behaviour goal setting, self-monitoring, instructions on the behaviour, and credible source. The risk-of-bias was “moderate” in eight studies and “high” in three.ConclusionThe present systematic review identified the components of theory-based nutrition and physical activity behaviour change interventions that may be beneficial for the treatment of overweight/obesity in breast cancer survivors. The strategies mentioned, in addition to reported behavioural models and BCTs, should be considered when developing weight-loss interventions for breast cancer survivors.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10552-023-01707-w.