Objective: To assess the clinical impact of living alone on weight gain in university students. Participants: This retrospective cohort study included 17540 male and 8854 female university students admitted to a national university in Japan. Methods: An association between living arrangement and the incidence of weight gain =10% and overweight/obesity (body mass index (BMI) =25 kg/m2) was assessed using multivariable-adjusted Poisson regression models. Results: Weight gain was observed in 1889 (10.8%) male and 1516 (17.1%) female students during 3.0 and 2.9 years of the mean observational period, respectively. Living alone was identified as a significant predictor of weight gain (adjusted incidence rate ratio of living alone vs. living with family: 1.24 [1.13-1.36] and 1.76 [1.58-1.95] in male and female students, respectively) and was also as a predictor of overweight/obesity. Conclusions: University students living alone were at a significantly higher risk of weight gain and overweight/obesity than those living with family.