Scholars and managers have raised the question of how to encourage employees to perform discretionary pro-environmental behaviors at work, termed organizational citizenship behaviors toward the environment (OCB-Es). This study examined how rationales for organizational sustainability relate to employees’ OCB-Es. We considered two rationales—eco-centric and organization-centric—and two sources—employees’ rationales and their perceptions of their employers’ rationales. Results from 489 working adults across a variety of organizations and occupations revealed that both eco-centric and organization-centric rationales at both individual and perceived organizational levels related to employees’ OCB-Es. Furthermore, we found interactive effects, such that employees’ perceptions of their organizations’ rationales were more important than their own rationales in determining OCB-Es. These findings contribute to a theoretical understanding of the complex and interrelated factors motivating employees to perform voluntary sustainability behaviors in organizations. In addition, our results are valuable for managers looking to increase employee sustainability behaviors.