We sought to gain a more holistic understanding of young adults' knowledge and perceptions around sustainable diets using a mixed methods online survey of 500 young adults (18-25 yrs.). Variables measured included food choice motivators (6-point importance scale), knowledge (ranking of efficacy of sustainable dietary behaviours), and several open-ended questions (responses assessed using inductive coding). Results show that environmental and health dimensions of sustainability are most salient for young adults. Decreasing food waste and consuming local and seasonal produce were rated as the most important behaviours for promoting a sustainable diet, while reducing consumption of sugary drinks, and reducing dairy consumption were rated as the least important. Cost was cited as the most important barrier to engaging in a sustainable diet. Young adults' knowledge around sustainable diets is mixed, but sustainability is an important motivator in their dietary choices. Results inform education and communication strategies that seek to mobilise young adults to make sustainable dietary decisions.