BACKGROUND: Physical activity is beneficial for preventing non-communicable and infectious diseases, such as pneumonia. Physical activity is also a potential protective factor for reducing coronavirus disease 2019 (COVID-19) severity. Conversely, outbreaks of respiratory viral infections are more likely to occur owing to group activities, opportunities for contact with individuals and vocalisations. Since the onset of the COVID-19 pandemic, several cases of COVID-19 outbreaks during various sports and exercise have been reported. However, the common causes underlying these outbreaks remain unclear. OBJECTIVE: The objective of this study is to identify the causes of COVID-19 outbreaks during sports and exercise using systematic review approach. METHODS: Our eligibility criteria were published articles reporting case investigation on COVID-19 outbreaks and the cause during sports and exercise. Studies such as reviews and observational studies without case investigations were excluded. PubMed, CINAHL, WHO COVID-19 Research Database and Ichushi Web were searched on 28 August 2023. The quality of included studies was rated using a quality criteria checklist adapted from a previous systematic review of influenza outbreaks. Vote counting of outbreak causes was performed by type of sports (team or individual). RESULTS: Twenty-one articles reporting 22 outbreaks were identified (quality: high, 9; medium, 9; and low, 3). The outbreaks were most frequently reported in fitness classes, followed by soccer. Most studies listed multiple causes of the outbreaks. The most common suspected cause of outbreaks in individual exercise, mostly from fitness classes, was poor ventilation and not wearing masks, followed by not maintaining physical distance and participation of individuals with some symptoms. In team sports, the most common cause was interaction outside the game, such as social events. CONCLUSIONS: This systematic review found a limited number of case investigations suggesting that COVID-19 outbreaks during sports and exercise may be associated with the inhalation of aerosols in indoor settings, interactions outside of team sports games and participation of individuals with some symptoms. Prevention strategies that focus on mitigating these issues may be effective at preventing sports and exercise-associated respiratory infectious diseases outbreaks. PROSPERO REGISTRATION NUMBER: CRD42023443158.