Objectives: The SAFE-Maintain study sought to evaluate the effect and acceptability of a physical activity counselling (PAC) session, versus a PAC session plus provision of a Fitbit (Charge HR®; PAC + F), on maintenance of physical activity levels 12 weeks following participation in a supervised exercise intervention. Design: Fifty-two women with stage II + breast cancer who had recently (within the previous 7 days) completed a 12-week supervised exercise program were randomised to the PAC or PAC + F group. Methods: Physical activity levels, including weekly minutes of total physical activity (min/week), daily step count (steps/day), and weekly minutes of moderate to vigorous physical activity (MVPA, min/week), were assessed using the Active Australia survey and Actigraph® GT3X+ accelerometers. Self-reported outcomes were assessed at baseline and 12-week follow-up, while objectively-measured outcomes were only available at 12-week follow-up. Results: Compared with the PAC group, the PAC + F group had higher self-reported MVPA and self-reported total activity (between-group mean difference: 78.2 [95% CI = −8.3, 164.9] min/week, p < 0.01, and 171.9 [95% CI = 46.1, 297.8] min/week, p < 0.01, respectively) at 12-week follow-up. Higher objectively-assessed MVPA (p = 0.03) and steps/day (p = 0.07) at 12-week follow-up was also observed in the PAC + F group compared with the PAC group. Most (>80%) of the PAC + F group reported high levels of Fitbit use and considered the device to be beneficial for physical activity maintenance. Conclusions: Findings suggest that activity trackers show promise as an effective, feasible and acceptable approach to support physical activity maintenance following completion of a supervised exercise intervention. Trial registration: Prospectively registered on the Australian and New Zealand Clinical Trials Registry (ANZCTR, Trial registration number: ACTRN12616000954426).