BACKGROUND: Plant-based diets, such as the Planetary Health Diet (PHDI) and the Mediterranean Diet (Med), offer notable advantages for human and planetary health. However, knowledge on the PHDI's benefits is limited, particularly in Southern European countries where the Med is culturally rooted and is an environmentally sustainable dietary pattern. OBJECTIVE: to evaluate the association of both PHDI and Med with mortality and assess their environmental burden in the adult population of Spain. METHODS: Data were taken from the study on Nutrition and Cardiovascular Risk in Spain (ENRICA) comprising 13,105 participants representative of the Spanish adult population. The PHDI score (0-140 points) was based on 15 food groups, while adherence to Med was assessed with the 14-item MEDAS score (0-14 points). Environmental impact was assessed using the SHARP-ID database (including greenhouse gas emissions and land use). Analyses were performed with Cox regression and adjusted for main confounders. RESULTS: During a mean 14.4-year follow-up, 1157 all-cause deaths occurred. The mortality hazard ratio (95 % CI) for the highest vs lowest tertile of the PHDI score was 0.78 (0.66, 0.91) but reached a plateau level at 90 points of PHDI. For the MEDAS, the corresponding results for the highest vs lowest tertile was 0.79 (0.68, 0.93) with a continuous inverse dose-response association. Adherence to some components of the PHDI (fruits, dairy, and unsaturated oils) and of MEDAS (nuts, and low consumption of soda and pastries) was independently and significantly associated with lower mortality. Results remained robust in sensitivity analyses. In terms of environmental impact, both plant-based diets had similar low footprints, with dairy and meat products being the largest contributors. CONCLUSION: In this large cohort of Spanish adults, higher adherence to the PHDI and MEDAS was similarly associated with lower all-cause mortality and showed comparable low environmental impact.