Introduction: A sustainable diet which is healthy and environmentally friendly provides the means of climate change mitigation in addition to promoting health of the population. There is an urgent need to have an indicator to measure if one’s diet is sustainable. This paper aimed to validate a newly developed Sustainable Diet Index (SDI) among young Malaysian adults. The SDI was developed based on the dietary guidelines of a sustainable diet. Methods: Five indicators (rice, animal-based food, plant-based food, food waste, and packaging) were included in the SDI. The index was validated via content validity, exploratory factor analysis (EFA), and confirmatory factor analysis (CFA) among young Malaysian adults. The dietary assessment tool used was an Android application named Sustainable Food Record. Results: Content validity showed fair to moderate correlations (0.331 - 0.816) between the indicators in the SDI. EFA produced five final factors with eight indicators in the index as follows: 1) fruits and vegetables; 2) dairy, eggs, and meat; 3) rice, cereals, and grain products; 4) food packaging; and 5) food waste management with strong factor loadings (0.760 – 0.984). All five factors with eight indicators were retained and proceeded with CFA. The fit indices from CFA demonstrated that the model was an absolutely fit. Conclusion: The validated SDI can be used as a tool to measure the sustainability of an individual’s diet in Malaysia, incorporating both health and environment considerations.