BACKGROUND: Current global food systems threaten human health and environmental sustainability. In 2019, the EAT-Lancet Commission on healthy diets from sustainable food systems defined the first global reference diet to improve both areas, but there is no consensus on how to quantify the EAT-Lancet reference diet as a diet index and its relation to mortality has not been widely studied. OBJECTIVE: To develop a new dietary index to quantify adherence to the EAT-Lancet diet and assess its association with mortality in a large population-based Swedish cohort. We also examined food components included in the index and their individual associations with mortality. DESIGN: We used the Malmö Diet and Cancer cohort (n = 22,421, 45-73 years at baseline). Dietary data were collected using a modified diet history method. The EAT-Lancet index was developed based on intake levels and reference intervals of 14 food components defined in the EAT-Lancet diet (0-3 points per component, 0-42 points in total). Associations with mortality were examined based on registers during a mean of 20 years of follow-up and were adjusted for potential confounders. RESULTS: Divided into five adherence groups, the highest adherence to the EAT-Lancet diet (≥23 points) was associated with lower all-cause mortality (HR: 0.75; 95% CI: 0.67, 0.85), cancer mortality (HR 0.76; 95% CI: 0.63, 0.92) and cardiovascular mortality (HR 0.68; 95% CI: 0.54, 0.84) than the lowest adherence (≤13 points). Several food components included in the index contributed to the observed reductions in mortality. CONCLUSIONS: We developed a new dietary index to investigate adherence to the EAT-Lancet diet. The findings indicate a 25% lower risk of mortality among those with the highest adherence to the EAT-Lancet diet, as defined using our index, which adds to the evidence base for the development of sustainable dietary guidelines.