This study aimed to investigate the relationships between event involvement and participation intention at an online marathon in Japan. We administered an online questionnaire via e-mail to runners at the 10th Ako City Marathon Online Event held in Ako from February 20 to March 7, 2021. Data were collected from 122 runners who were at least 18 years old. To examine the relationships between event involvement and participation intention, a hierarchical multiple regression analysis was conducted with participation intention as the dependent variable, socio-demographics (gender, age, place of residence, family and household structure) as the independent variables in Step 1, and event involvement (attractiveness, centrality, social bonding) as the independent variables in Step 2. The findings showed that age was negatively related to the dependent variable. In Step 2, as in Step 1, age was found to be negatively related to participation intention, while attraction and social bonding for event involvement were positively related to participation intention. The coefficient of determination increased significantly when the event involvement was added as the independent variables, indicating its significance in explaining participation intention..