Objective: University students are highly sedentary, increasing their risk of poor health outcomes. This study aimed to co-design and pilot a behavioural gain-framed nudge-based intervention to reduce university students' sedentary behaviour by breaking up long periods of sitting every 30 minutes. Methods: Experienced-based co-design was used to conduct three workshops with university students and staff (n = 11) to develop and create the intervention (UC30). A mixed-methods pilot pre-post study investigated the effectiveness of the intervention in a university cohort (n = 60) over one semester. Semistructured interviews (staff, n = 6; students, n = 3) and student survey responses (n = 43) were used to determine the primary outcomes of acceptability and feasibility. The secondary outcome measure was selfreported sedentary time (Past-day Adults' Sedentary Time-University questionnaire, minutes/day). Results: Co-designed resources were simple to implement for staff and informative and influential for students. The qualitative analysis revealed three themes relevant to both students and staff: 'delivery mode', 'academic engagement' and 'education and impact'. Total self-reported sedentary time did not decrease among students; however, there was a 51-minute daily reduction (95% confidence interval: -121, 19) in sitting-for-study post-intervention. Conclusion: Co-designed behavioural nudge-based resources as part of learning activities to reduce sedentary behaviour in university students may be effective in reducing sedentary time during study, improving health and learning outcomes, indicating a larger trial is warranted.