This study examines the relationship between departmental managers' budget-related behaviors and departmental performance in nonprofit organizations under varying degrees of task interdependence—specifically, when task interdependence is low (pooled task interdependence) and when it is high (reciprocal task interdependence). Research on departmental managers' budget-related behaviors has primarily targeted for-profit organizations and has employed various independent variables, dependent variables, and organizational contingency variables, yet studies utilizing moderating variables to account for differences in the influence between independent and dependent variables have been virtually nonexistent. This paper represents an exploratory attempt to overcome the aforementioned limitations. Through a review of prior research, common factors constituting managers' budget-related behaviors were identified and applied to nonprofit organizations to expand the research on managers' budget-related behaviors. Budget-related behaviors were used as the independent variable, departmental performance in nonprofit organizations as the dependent variable, and task interdependence as the moderating variable to derive hypotheses. Hypothesis testing employed subgroup analysis to verify the existence of the moderating effect of departmental task interdependence. The results showed that the moderating effect of task interdependence was present in the relationships between all independent variables and departmental performance except for the budget participation variable, suggesting that task interdependence as a moderating variable plays an important role in nonprofit organizations. By conducting research with such objectives, it is expected that each department in nonprofit organizations can first accurately assess its task situation and then consider budget-related behavioral factors appropriate to that situation, thereby contributing to improved departmental performance. Furthermore, efforts to identify variables related to or influencing efficiency improvement in nonprofit organizations are considered useful even if such research is exploratory in nature.