The novel coronavirus disease (COVID-19) pandemic has spread throughout the world. Poor mental health has been reported among healthcare professionals responding to COVID-19. However, no study has examined the impact of COVID-19-related workplace bullying or patient aggression on the mental health of healthcare professionals during the COVID-19 outbreak. This study examined the prevalence of COVID-19-related workplace bullying and patient aggression and its association with psychological distress among healthcare professionals during the COVID-19 outbreak in Japan. This was a cross-sectional study conducted from May 22 to 26, 2020, inviting participants (n = 1,421) from an online survey of full-time employees. We limited the sample to healthcare professionals for further analyses. Using an online self-report questionnaire, workplace bullying and patient aggression related to COVID-19 was measured using nine items with dichotomous response options. Psychological distress was measured using the Japanese version of Brief Job Stress Questionnaire. Among 1,032 participants (72.6%) who completed the survey, 111 healthcare professionals were identified. Among them, 19 participants (17.1%) had experienced any COVID-19-related workplace bullying or patient aggression: 11 participants (9.9%) had experienced any workplace bullying and 12 participants (10.8%) had experienced any patient aggression. Multiple linear regression analysis showed that any bullying or patient aggression related to COVID-19 significantly correlated with psychological distress. It was suggested that a non-negligible proportion of participants experienced workplace bullying or patient aggression related to COVID-19. Preventing and reducing workplace bullying and patient aggression may be effective in improving mental health of healthcare professionals during the COVID-19 outbreak.