Objective: Transition to university is a vulnerable period for discontinuing regular physical activity that can have implications for students’ physical and psychological health. Accordingly, it is imperative to find and implement cost and time-effective interventions to mitigate the consequences of this transition. Intervention research has shown mental contrasting is an effective means of promoting various health-enhancing behaviors including physical activity. However, the efficacy of this intervention approach may be bolstered by targeting affective judgments (e.g., enjoyable–unenjoyable), which exert greater influence on physical activity behaviors compared to health-related instrumental judgments (e.g., useful–useless). The purpose of this study was to compare the relative efficacy of a mental contrasting intervention that targeted affective judgments associated with physical activity, in relation to a mental contrasting intervention that targeted instrumental judgments, and a “standard” mental contrasting intervention (with no modifications). Method: Using a 3-arm parallel randomized controlled trial design (ClinicalTrials.gov Number NCT02615821), 110 insufficiently active, female, university students were randomly assigned to an affective, instrumental, or standard mental contrasting intervention following simple randomization procedures. Assessments were conducted at baseline and 4 weeks postintervention. Results: Participants in the affective mental contrasting condition displayed higher levels of moderate-to-vigorous physical activity than those in the instrumental or standard comparison conditions, F(2, 90) = 3.14, p < .05, ηp2 = 0.065. Conclusion: Overall, affective mental contrasting has the potential to represent a low-cost and time-efficient intervention that may help insufficiently active, female students increase activity or attenuate declining levels of moderate-to-vigorous physical activity that occurs during university.