Students transitioning to university face many challenges, including decreased physical activity and psychological wellbeing. Mental contrasting, a self-regulation strategy that involves setting a goal, then associating prospective positive outcomes with present obstacles related to that goal, may be effective to boost physical activity and wellbeing. Research shows that mental contrasting may be particularly effective when framed with affective messaging (e.g., exercise is enjoyable). The purpose of this study was to examine the efficacy of 4 weeks of affective mental contrasting, compared to goal setting alone, on physical activity and psychological wellbeing among first year university students. In a randomized controlled trial over a 4-week period, 114 first year university students were guided through either goal setting alone, or goal setting plus affective mental contrasting, focused on physical activity. Physical activity and psychological wellbeing were measured via questionnaire at baseline and 4 weeks. Participants in both groups significantly increased their physical activity, F (1, 85) = 50.13, p < .001, partial eta squared = .37 and psychological wellbeing, F (1, 82) = 5.14, p < .05, partial eta squared = .06, but between-group differences were not significant. An exploratory analysis showed that among participants who were active for more than 60 minutes per week at baseline, affective mental contrasting was more effective than goal setting alone at increasing physical activity, F (1, 54) = 4.73, p = .034, partial eta squared = .08. Both goal setting and affective mental contrasting are effective strategies to increase physical activity among university students.