Many studies have shown that HR practices have a positive impact on organizational ambidexterity; however, few have examined both employees' motivation for ambidexterity together with their ability to achieve it, and even fewer have concentrated on organizations in the nonprofit sector. Drawing on the ability-motivation-opportunity framework, this study examines employees' ambidextrous behaviors as mediator between high-involvement HR practices and organizational ambidexterity. It addresses the lack of in-depth insights by examining ambidexterity at both the individual and the firm level, and addresses the lack of focus on nonprofit organizations by undertaking the study in a leading nonprofit hospital in Egypt. Based on a sample of 130 healthcare personnel, the results show that a "bundle" of high-involvement HR practices is positively related to organizational ambidexterity, and that employees' ambidextrous behaviors mediate the relationship between high-involvement HR practices and organizational ambidexterity. The results also highlight the value of high-involvement HR practices in encouraging employees' ambidextrous behavior through staffing, teamwork, and workgroup training, thus helping the organization to achieve ambidexterity.