Drawing on the theory of institution, this research investigated the effects of institutional responses, employees' sustainability behavior, and sustainability transformational leadership on sustainability performance at seaports. Data were collected from 296 employees of port corporations in Taiwan, and a moderated mediation model was developed. Using exploratory and confirmatory factor analyses, this study identified four port institutional response dimensions: coercive policy, normative training, communication, and motivation. The results showed that sustainability transformational leadership, employees' sustainability behavior, and port institutional responses positively affect sustainability performance. Employees' sustainability behavior plays a mediating role between sustainability transformational leadership and sustainability performance. Specifically, this study found that port institutional responses play a moderated mediation effect on the relationship between sustainability transformational leadership and sustainability performance through employees' sustainability behavior. Lastly, implications for port sustainability practices and institution theory are discussed.