Abstract Background Most adults fail to meet global physical activity guidelines set out by the World Health Organization. In recent years, behavioural economic principles have been used to design novel interventions that increase physical activity. Immediate financial rewards, for instance, can motivate an individual to change physical activity behaviour by lowering the opportunity costs of exercise. This systematic review will summarise the evidence about the effectiveness of financial incentive interventions for improving physical activity in adults. Methods We will search MEDLINE, Embase, Cochrane Central Register of Controlled Trials, the Cumulative Index to Nursing and Allied Health Literature, Web of Science, Scopus, PsycINFO, EconLit, SPORTDiscus, the National Health Service Economic Evaluation Database, ClinicalTrials.gov and the World Health Organization International Clinical Trials Registry Platform from inception using a comprehensive, electronic search strategy. The search strategy will include terms related to ‘financial incentive’ and ‘physical activity’. Only randomised controlled trials that investigate the effect of financial incentives on physical activity in adult populations and that are written in the English language will be included. Two review authors will independently screen abstracts and titles, complete full text reviews and extract data on objective and self-reported physical activity outcomes. The authors will also assess the study quality using the Cochrane risk of bias tool and provide a systematic presentation and synthesis of the included studies’ characteristics and results. If more than two studies are sufficiently similar in population, settings and interventions, we will pool the data to conduct a meta-analysis. If we are unable to perform a meta-analysis, we will conduct a narrative synthesis of the results and produce forest plots for individual studies. Our subgroup analyses will examine the differential effects of an intervention in healthy populations compared to populations with disease pathology and compare the effects of interventions using financial rewards to interventions using financial penalties. Discussion This systematic review will determine the effectiveness of positive and negative financial incentives on physical activity in adults. Findings will help inform the development of public health interventions and research in this field. Systematic review registration PROSPERO 2017: CRD42017068263