["Applied Psychology: Health and Well-Being, Volume 18, Issue 3, June 2026. ", "\nAbstract\nAdopting a low‐emission diet and choosing low‐emission transportation modes are among the most effective strategies for mitigating one's individual impact on climate. However, interventions targeting these behaviors often fall short because they focus primarily on motivation, neglecting volitional processes. Guided by the Health Action Process Approach, this double‐blind randomized parallel trial tested whether a digital intervention addressing both motivational and volitional determinants of behavior was more effective in reducing diet‐ and transportation‐related carbon footprints than an intervention addressing motivation only. The intervention was delivered via the GROW app, which included two versions: a motivational version (goal setting and feedback) and a motivational + volitional version (additional techniques such as action planning and problem solving). Participants (N = 226; 97% students) used either version of the app for 5 weeks, reporting daily animal‐based food consumption (e.g., red meat and poultry) and transportation behaviors (e.g., bike and car). Psychological determinants (e.g., action planning and action control) were measured weekly. Multilevel models showed an overall reduction in diet‐related carbon footprint (B = −0.1, 95% CI [−0.02, −0.01]); however, this decrease did not differ between intervention groups. No changes emerged for transportation‐related or total carbon footprints. Action planning and action control emerged as the strongest correlates of lower diet‐related carbon footprint. The findings show preliminary indication of the efficacy of digital behavior change interventions for reducing diet‐related carbon footprint. Structural measures addressing opportunities and barriers in the physical environment may be needed to reduce transportation‐related emissions.\n"]