Although the relationships between procedural justice climate and organizational citizenship behaviors have been examined in recent years, little research has explored the mechanism by which procedural justice climate shapes individual employee prosocial behaviors in the workplace. The purpose of this study was to examine the mediating role of a group-level cooperative norm on the relationships between the group-level procedural justice climate and individual-level organizational citizenship behaviors. The survey involved 45 work groups in four different industry fields in Taiwan, including manufacturing, technology, banking, and insurance, and each of the groups was composed of one supervisor and three subordinates. Cross-level analyses using hierarchical linear modeling (HLM) indicated that the cooperative norm fully mediated the relationship between procedural justice climate and individual helping behaviors. Procedural justice climate indirectly affects individual helping behaviors through their effects on the cooperative norm.