Obesity in pregnancy is rising and is associated with severe health consequences for both the mother and the child. There is an increasing international focus on guidelines to manage the clinical risks of maternal obesity, and for pregnancy weight management. However, passive dissemination of guidelines is not effective and more active strategies are required for effective guideline implementation into practice. Implementation of guidelines is a form of healthcare professional behaviour change, and therefore implementation strategies should be based on appropriate behaviour change theory. This systematic review aimed to identify the determinants of healthcare professionals' behaviours in relation to maternal obesity and weight management. Twenty-five studies were included. Data synthesis of the existing international qualitative and quantitative evidence base used the Theoretical Domains Framework to identify the barriers and facilitators to healthcare professionals' maternal obesity and weight management practice. The domains most frequently identified included 'knowledge', 'beliefs about consequences' and 'environmental context and resources'. Healthcare professionals' weight management practice had the most barriers compared with any other area of maternal obesity practice. The results of this review will be used to inform the development of an intervention to support healthcare professional behaviour change.