We aimed to investigate the lifestyle factors influencing weight gain among university students in Japan during the mild lockdown imposed due to the novel coronavirus disease pandemic. In this cross-sectional study, we conducted a questionnaire survey of students who underwent health examinations at Nagasaki University in 2021. Students reporting a weight gain of ≥3 kg were included in the weight gain group; the remaining students were included in the non-weight-gain group. Fisher’s exact test and binary logistic regression were performed to determine the association between weight gain and each lifestyle factor. We included 3059 respondents (response rate: 45.7%), and 9.5% of them reported a weight gain of ≥3 kg. The following factors were associated with weight gain (odds ratio (95% confidence interval), p value based on Fisher’s exact test): dining out for four times or more/week (2.16 (1.40, 3.32), p = 8.7 × 10⁻⁴) and gaming time of ≥4 h/day (2.26 (1.45, 3.47), p = 2.4 × 10⁻⁴). Binary logistic regression among the four highest odds ratios revealed that after adjusting for other factors, frequent dining out and prolonged gaming time were significantly associated with weight gain in students during the mild lockdown.