This study explored whether lifestyle therapy that promoted adherence to a Mediterranean-style diet as a treatment for depression led to environmental co-benefits. Participants (N = 75 complete case) were Australian adults in the Curbing Anxiety and Depression using Lifestyle Medicine (CALM) non-inferiority, randomised controlled trial, which showed that lifestyle therapy was non-inferior to psychotherapy in reducing depressive symptoms, when delivered in group format via video conferencing over an 8-week treatment period. In this secondary analysis, we hypothesised that the lifestyle arm would be superior to the psychotherapy arm in reducing environmental impact of self-reported diet over time. Dietary intake derived from Food Frequency Questionnaires at baseline and 8 weeks were transformed into environmental impact scores by calculating Global Warming Potential (GWP)*. GWP* was calculated for total dietary intake and distinct food groups (Australian Dietary Guidelines [ADG] and NOVA classifications). Within-arm changes in GWP* over time were calculated using median difference. Neither arm showed significant changes. Between-arm differences in percentage change in GWP* scores over time were analysed using Generalised Estimating Equations (GEE) models. No between-arm difference for total GWP* score was found (β=11.06 [-7.04 29.15]). When examining distinct food groups, results were mixed. These novel findings contribute to the sparse evidence base that has measured the environmental impact of diets in a clinical trial context. Whilst lifestyle therapy that reduced depressive symptoms did not have clear environmental benefits relative to psychotherapy, nutritional counselling that focuses on the environmental impact of food choices may drive more pronounced planetary co-benefits.