Background Population level physical activity generally does not meet recommended targets. Compared with private motor vehicle users, public transport users tend to be more physically active and financial incentives may encourage more public transport use, but these relationships are under-investigated. This paper describes the protocol of a randomised controlled trial that aimed to determine the effect of financially incentivising public transport use on physical activity in a regional Australian setting.

Methods Get BusActive! is a 9.5-month single-blinded randomised controlled trial. A convenience sample of Tasmanians aged ≥15 years will be randomised to a 14-week incentive-based intervention (bus trip target attainment rewarded by bus trip credits and weekly supportive text messages) or an active control following baseline measures and will be followed up ∼24 weeks later (maintenance phase). Both groups will receive written physical activity guidelines. The primary outcome is change in accelerometer-measured steps/day from baseline to immediately post intervention phase and maintenance phase. Secondary outcomes are change in: smartcard-measured bus trips/week; measured and self-reported minutes/week of physical activity and sitting; transport-related behaviour (using one-week travel diary), perspectives (e.g. enablers/barriers) and costs; health. Linear mixed model regression will determine group differences. Participant-level process evaluation will be conducted and intervention cost to the public transport provider determined.

Conclusion Get BusActive! will fill an important knowledge gap about the causal relationship between financially incentivised public transport use and physical activity—the findings will benefit health and transport-related decision makers.

Trial registration ACTRN12623000613606.

Universal trial number U1111-1292-3414.