Purpose To compare differences in happiness and stress and related factors between pre-clinical and clinical year medical students during the coronavirus disease 2019 (COVID-19) pandemic. Methods A cross-sectional study was conducted in the Faculty of Medicine, Chiang Mai University, Thailand. All undergraduate medical students were requested to voluntarily respond to an electronic survey. Demographic data, related factors of happiness and stress, scores from the Thai version of the Oxford Happiness Questionnaire (Thai-OHQ), and Thai Stress Questionnaire (Thai-ST5) were collected. Results There were 369 responses, 64.8% from preclinical students and 35.2% responses from clinical students, and 53.9% were women. The mean age of the participants was 20.62±1.81 years. The most frequent platforms that the students used to track COVID-19 information were Facebook 43.9% and Twitter 43.4%. Both groups had a low level of stress. No difference was found in the Thai-OHQ score (p=0.323) and the Thai-ST5 score (p=0.278). With multivariable analysis, two factors significantly related to the happier students included higher health satisfaction scores (p<0.001) and maintaining an exercise program during the COVID-19 pandemic (p=0.015). Conclusion There was no difference in the happiness and stress levels between the two groups during the first outbreak of COVID-19 in Thailand. To increase happiness, promoting awareness of health satisfaction and regularity of exercise for the medical students should be initiated. To direct the information during a disease outbreak such as the COVID-19 pandemic, Facebook, and Twitter are the primary platforms to use.