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Background Dietary patterns that limit or avoid animal products contribute fewer greenhouse gases and require less energy than diets containing more animal products. However, quantifications of these outcomes on a vegan diet in real-life conditions still need to be explored.Methods In a randomised controlled trial, a low-fat vegan diet supplemented with soybeans led to a significant reduction in hot flashes in postmenopausal women. In this secondary analysis of that study, we hypothesised that the intervention diet would have a reduced environmental impact, greenhouse gas emissions (GHGEs) and cumulative energy demand (CED). 3-day diet records were analysed at baseline and at the end of the 12-week study, and we linked dietary intakes to environmental footprints.Results Compared with no change in the omnivorous control group, we found that the vegan group had a decrease in GHGEs (p <0.001) of 35% (−662.7 g CO 2-eq/person-day) and a decrease in CED (p =0.002) of 34% (−4956 kJ/person-day). These reductions in environmental impact were primarily due to reduced consumption of meat and dairy products.Conclusion Adoption of a soybean-supplemented low-fat vegan diet significantly reduced cumulative energy demand and GHGEs, important drivers of climate change.Trial registration number NCT04587154.