Objectives: The coronavirus disease 2019 pandemic and its associated societal responses are anticipated to have wide-ranging effects on youth development and mental health. Depression, anxiety, and attention deficit hyperactivity disorder (ADHD) are the three most common mental health problems among university students. Many factors that can threaten mental health during the outbreak affect these three conditions, as well as sleep conditions, in undergraduate students. Thus, determining how these abrupt changes in students’ circumstances impact their mental health is very important from a public health perspective. Methods: We investigated the usual conditions and changes in ADHD symptoms during the outbreak, in relation to depressive and sleep-related symptoms among undergraduate students. A total of 252 students, primarily juniors, completed the online survey. Results: The results showed that 12% of the students exceeded the cut-off score of the ADHD questionnaire before the pandemic.Approximately 6%–21% of the university students, especially those with ADHD traits, rated their ADHD behaviors as worse during the outbreak than that before the outbreak. Conclusion Female students and undergraduates with ADHD traits are more susceptible to experiencing further deterioration of ADHD (inattention) symptoms during the pandemic. In cases where it is difficult to intervene with ADHD symptoms, approaching circadian rhythm or depression will be of considerable clinical use.