Abstract Background Physical inactivity is a leading contributor to the development of non-communicable diseases including cardiovascular disease, diabetes, and cancer. Public health messages that advocate activity guidelines with specific duration thresholds (e.g., 30 minutes per day) have had limited success. This approach may risk an ‘all or nothing’ mindset whereby people feel exercise is not worthwhile unless they can meet recommended guidelines. World Health Organization guidelines recognise that any activity is better than none. In line with this, the present study aimed to test the effectiveness of two messages that referred to relative rather than absolute increases in physical activity: ‘Sit less, move more’ and ‘Add 2000 steps to your day’. Methods A representative sample of 7995 Australian adults responded to three surveys over six-weeks, with random assignment to a control condition (non-physical activity message) or one of two physical activity message conditions: Sit less, move more or Add 2000 steps to your day at each time point. At all three time points, respondents were exposed to their allocated message and reported their frequency of enacting the recommended behaviours. To account for attrition over the surveys, an intention-to-treat approach was adopted. Two generalised estimating equations models were run to examine time-by-condition interactions for the enactment of each message. Results Compared to control, respondents in the Add 2000 steps to your day condition significantly increased their enactment of adding 2000 steps to their day over time (B=.161, p=.011). Those in the Sit less, move more condition did not significantly differ from control in the frequency of enacting this message. Conclusions Promoting a specific physical activity goal that was relative to baseline activity increased the enactment of the recommended behaviour. Messages seeking to promote physical activity may be more effective if they include goals that are clear and feasible. Key messages • A message encouraging people to add 2000 steps to their day successfully increased physical activity. • Promoting achievable goals in physical activity messages might be a useful public health strategy.