Background: There is evidence that brief physical activity interventions by health professionals can increase physical activity levels. In addition, there is some evidence that simply measuring physical activity alone can increase physical activity behaviour. However, preliminary work is required to determine the effects of potential measurement frequency. The aim of this pilot study was to examine whether frequency of physical activity measurement, with very brief advice from a physiotherapist, influenced objectively measured physical activity in insufficiently active adults. Methods: Using concealed allocation and blinded assessments, eligible participants (n = 40) were randomised to a lower-measurement-frequency (baseline and 18-weeks) or higher-measurement-frequency group (baseline, 6, 12 and 18-weeks). The primary outcome was daily minutes of moderate-to-vigorous physical activity (accelerometry). Secondary outcomes included functional aerobic capacity (STEP tool), quality-of-life (AQoL-6D), body mass index, waist circumference, waist-to-hip ratio and blood pressure. Results: Between-group comparisons were not significant in intention-to-treat analyses. However, there was a trend for the higher-measurement-frequency group to complete more daily minutes of moderate-to-vigorous physical activity at 18-weeks (mean difference 19.6 vs - 11.9 mins/week, p = 0.084), with a medium effect size (Cohen's d = 0.58). This was significant in per-protocol analysis (p = 0.049, Cohen's d = 0.77). Within-group comparisons indicated both groups increased their aerobic fitness (p ≤ 0.01), but only the higher-measurement-frequency group decreased their waist circumference (mean decrease 2.3 cm, 95%CI 0.3-4.3, p = 0.024), diastolic blood pressure (mean decrease 3.4 mmHg, 95%CI 0.03-6.8, p = 0.048) and improved their quality-of-life for independent living (mean increase 3.3, 95%CI 0.2-6.4, p = 0.031). Conclusion: Very brief physical activity interventions by physiotherapists may be an efficient approach to increase physical activity in community-dwelling adults. A larger trial is warranted. Trial registration: Australian New Zealand Clinical Trials Registry (ANZCTR): ACTRN12616000566437, http://www.ANZCTR.org.au/ACTRN12616000566437.aspx, registered 2 May 2016.