Promoting healthy and sustainable diets is needed to address global health and environmental challenges. (1) Young Australian adults (aged 18-25 years) represent a key group for dietary intervention as they have the poorest diet quality among all adults. (2) Their motivation towards climate action provides a unique opportunity to encourage more healthy and sustainable eating behaviours. (3) This study aimed to evaluate the feasibility and effectiveness of an app-based healthy and sustainable dietary intervention in young Australian adults (n=36). A four-week single-arm intervention was conducted on the DeakinWELLBEING mobile application which included a range of features to support adherence to a healthy and sustainable diet. This includes healthy and sustainable eating content, e.g. recipes, a habit tracker and a healthy eating challenge. Participant data were collected at baseline (0 wk), post-intervention (4 wk), and at follow-up (8 wk). Feasibility (retention rate), engagement (users per day, habits completed and clicks to links) and self-reported acceptability (Post-Study System Usability Questionnaire (PSSUQ)) (4) were measured post-intervention. Significant differences in sustainable food literacy scores and food group intakes were assessed using a validated scale(5) and a 37-item food checklist, respectively, at each timepoint. Retention rate was 78%. Mean PSSUQ score was 5.4±0.9 out of 7, suggesting acceptable usability of the DeakinWELLBEING mobile application. On average, there were 14 unique active users per day. Sustainable food knowledge and action intent/action strategies significantly improved post-intervention (p<0.001) and at follow-up (p<0.001). Food skills significantly improved post-intervention (p=0.014), whereas attitudes towards sustainable eating improved at follow-up (p=0.002). Wholegrain and legume intakes increased post-intervention by 71% (95%CI: 6, 175%) and 124% (95%CI: 39, 259%), respectively. These food groups also increased from baseline to follow-up by 80% (95%CI: 11, 193%) and 101% (95%CI: 24, 225%), respectively. Nut intake increased post-intervention by 86% (95%CI: 14, 201%). The intervention was considered feasible, acceptable and effective in improving sustainable food literacy and adherence to healthy and sustainable diets among young Australian adults. These results can inform larger randomised controlled trials, health promotion initiatives and the development of tools to empower healthy and sustainable dietary habits. The trial was prospectively registered in the Australian New Zealand Clinical Trials Registry (Registration ID: ACTRN12625000335493p) on 22nd April 2025.