Livestock production systems are associated with climate change, land degradation, and animal welfare concerns, while overconsumption of animal-sourced foods is a major driver of human disease. Thus, shifting towards plant-rich diets is expected to deliver benefits for human health, the environment, and animal welfare. Nevertheless, diets high in animal products are flourishing, especially in high-income countries. Here, we take a novel inter-disciplinary approach to evaluating sustainability of diets by assessing five common plant-rich diets (Mediterranean, flexitarian/semi-vegetarian, vegan, vegetarian, and pescatarian) on two metrics. First, we established each diet's environmental, human health, and animal welfare impacts, using quantitative data sourced from a review of the literature, including life cycle assessments. Second, we evaluated the human factor by surveying current consumer dietary preferences (i.e., which diet participants had followed over the past week) and the likelihood of adopting each plant-rich diet in the future, among a sample of Australian adults (n = 253). Combining the results from the review and the survey in a Behavior Prioritization Matrix (BPM), the Mediterranean diet was shown to have the greatest projected positive impact, followed by the vegetarian diet. This study is the first to combine assessments of sustainable diets on the three dimensions of environment, human health, and animal welfare with probability of adoption. Our findings highlight the necessity of assessing plant-rich diets through a holistic lens when identifying target diets to promote, in order to support sustainable food systems in high-income countries.