Background: Adopting healthy weight-related behaviours in emerging adulthood has long-term health benefits. Digital interventions using game-elements have the potential to improve diet and physical activity outcomes. Universities are influential settings in shifting emerging adults' behaviour and can deliver health-promoting interventions to young adults. Objectives: To investigate the effect of an online intervention using quiz-games on nutrition knowledge, diet quality, physical activity and sitting time in a sample of university students. Design: Randomised, single-blind, controlled trial. Setting: Two UK Universities including a total of 88 students from different faculties. Method: The intervention group (n = 50) had access to 10 quiz-games and a website for 10 weeks. The control group (n = 38) received no such resources. Main outcomes were nutrition knowledge, diet quality, physical activity and sitting time assessed by self-administered questionnaires. Frequency and performance of the quiz-games played, and the effect of time (days) on performance after playing a quiz-game for the second time, were also investigated. Results: No significant differences were found between groups over time in any of the outcomes. The outcomes remained non-significant when engagement rates with the quiz-games were considered. Among the 35 game-players (n = 15 did not play any game), 15 played at least one quiz-game. Frequency of playing a quiz-game by all students ranged from 43 to 17 times, and mean scores (% of correct answers) ranged from 55% to 82%. When repeating a quiz-game within 8 days, the performance (score) was improved while after that point, performance remained the same or decreased. Conclusion: Online game-based interventions can be easily implemented in university settings. However, further research is needed on their design to enhance engagement and identify key factors affecting students' behaviour to increase their effectiveness.