Background: Smoking during pregnancy is the most important preventable cause of adverse pregnancy outcomes, yet smoking cessation support (SCS) is inconsistently provided. The MOMHQUIT intervention was developed to address this evidence-practice gap, using the Behaviour Change Wheel method by mapping barriers to intervention strategies. MOHMQuit includes systems, leadership and clinician elements. This implementation trial will determine the effectiveness and cost-effectiveness of MOHMQuit in improving smoking cessation rates in pregnant women in public maternity care services in Australia; test the mechanisms of action of the intervention strategies; and examine implementation outcomes. Methods: A stepped-wedge cluster-randomised design will be used. Implementation of MOHMQuit will include reinforcing leadership investment in SCS as a clinical priority, strengthening maternity care clinicians’ knowledge, skills, confidence and attitudes towards the provision of SCS, and clinicians�