Previous research suggest that gratitude interventions are effective in improving mental well-being, which might be beneficial to university students during the COVID-19 pandemic. This quasi-experimental study sought to investigate if a gratitude intervention will lead to higher mental well-being of university students during the COVID-19 pandemic. Participants (N = 47) mental well-being was assessed before and after 10 weekly gratitude reflection journals and statistically compared with a control group (N = 40). An analysis of covariance (ANCOVA) was used to analyze the data. The treatment group showed significantly higher well-being after the gratitude intervention compared with the control group (Cohen’s d = 0.74). The treatment group significantly increased (Cohen’s d = 0.35) and the control group significantly decreased (Cohen’s d = −0.41). Gratitude interventions may be effective in improving the mental well-being of university students even during a crisis such as the COVID-19 pandemic. Gratitude interventions seem suitable for improving mental well-being for temporary mental challenges of university students such as a pandemic or other forms of crisis.