This review aimed to identify whether motivational interviewing (MI) (a counselling approach for supporting behaviour change [BC]) helps to reduce bodyweight and BMI in an adult obesity context. This included evaluating effectiveness of MI interventions within this population and reporting the methodology used, including theoretical underpinnings and identification of BC and MI techniques. Eight databases were searched using controlled vocabulary. Eligible studies included adults with obesity (BMI ≥30 kg/m2), author-reported interventions using MI aiming to reduce body weight or BMI, and comparator groups not receiving an MI intervention. Data extraction and quality appraisal tools were used to identify study characteristics, intervention content was coded for techniques, and random-effects meta-analysis were conducted to investigate effects. Meta-analysis of 12 studies indicated no overall pooled effect on bodyweight and BMI outcomes between intervention and control groups (SMD = −0.01 [95%CI −0.13 to 0.12, P = .93]). Findings were limited by multiple sources accounting for risk of bias, and poor reporting of intervention fidelity and content. Intervention and control content descriptions indicated similar techniques, with social support, goal setting (behaviour) and self-monitoring of behaviour occurring most frequently across both. Findings do not contribute additional evidence for MI use in this context, however methodological limitations were identified which must be resolved to better identify the intervention effects on obesity-related outcomes.