Objectives This study examined the mediating effects of learned helplessness in relationship among the just world beliefs, workplace bystander and workplace pro-bullying. Methods For this purpose, an online survey was conducted on 308 male and female workers aged 20 to 60 across the country who witnessed workplace bullying. Based on the collected data, descriptive statistics and correlation analysis were performed. In addition, multiple regression analysis was performed to examine the mediating effect of learned helplessness and just world beliefs, and bystander behavior and pro-bullying bystanders in workplace harassment, and significance of mediating effects was verified through the Sobel test. Results There was a significant negative correlation between the just world beliefs and learned helplessness and there were significant positive correlations among the just world beliefs, learned helplessness, and pro-bullying behavior. There was a significant partial mediating effect of learned helplessness when the just world belief of people around workplace bullying leads to pro-bullying behavior, but no significant mediating effect was derived of learned helplessness between the just world beliefs and bystander behavior. Conclusions The results of this study are interpreted to show that the just world beliefs and learned helplessness are important variables in the workplace bystander that trigger pro-bullying behavior in the bystander. Limitations of this study and suggestions for further studies were discussed.