OBJECTIVES: Habits may enhance the maintenance of physical activity. The aim of this study is to examine if reinforcing implementation intentions increases habit strength and thus physical activity. DESIGN: Participants (N = 186) were randomized into one of three intervention conditions (imagery vs. implementation intentions vs. combined; implementation intentions and imagery) and a control condition. METHODS: Participants were instructed to engage in a physical activity that they would like to start doing, or do more of, over a four-week period. Participants completed measures of physical activity, habit strength, and imagery use (imagery and combined conditions only) pre-intervention, weekly during the intervention, post-intervention, and a 12-week follow-up. RESULTS: Habit strength increased at week three (p < .001) for those in the combined condition, which was maintained through post-intervention and increased at follow-up (Ps > .05). In the imagery condition, habit strength increased at post-intervention (p = .003) and was maintained at follow-up. Physical activity increased for the combined condition from week two (p < .001) of the intervention, continuing to increase at weeks three (p = .003) and four (p < .001). CONCLUSIONS: Imagery may be an effective intervention to support habit formation. Reinforcing implementation intentions with mental imagery may support habit formation for physical activity behaviour.