Aim: This systematic review aimed to describe behaviour change theories and techniques used to inform nutrition interventions for adults undergoing bariatric surgery. Methods: A systematic search was conducted across PubMed, PsycInfo, CENTRAL, EMBASE and CINAHL from inception until 09 March 2021. Eligible studies were randomised controlled trials involving nutrition interventions performed by a healthcare provider, to adults that were waitlisted or had undergone bariatric surgery and received a nutrition intervention explicitly informed by one or more behaviour change theories or behaviour change techniques. Screening was conducted independently by two authors. Behaviour change techniques were examined using the behaviour change technique taxonomy version one which includes 93 hierarchical techniques clustered into 16 groups. Quality of included studies was assessed using Cochrane risk of bias 2.0. Results: Twenty-one publications were included, involving 15 studies and 14 interventions, with 1495 participants. Bias was low or had some concerns. Two interventions reported using behaviour change theories (transtheoretical model and self-determination theory). Thirteen behaviour change technique taxonomy groupings and 29 techniques were reported across 14 interventions. Common techniques included ‘1.2 Problem solving’ (n = 9 studies), ‘3.1 Social support (unspecified)’ (n = 9 studies), ‘1.1 Goal setting (behaviour)’ (n = 6 studies) and ‘2.3 Self-monitoring of behaviour’ (n =- 6 studies). Conclusion: While behaviour change techniques have been included, behaviour change theory is not consistently reported and/or adopted to inform nutrition interventions for adults undergoing bariatric surgery. Integrating behaviour change theory and techniques in nutrition interventions is important for researchers and bariatric surgery teams, including dietitians, to effectively target behaviours for this population.