Knowledge hiding occurs when employees deliberately withhold knowledge requested by others, potentially hindering knowledge exchange and collaboration. Drawing on social exchange theory and social information processing theory, this study examines how and when leaders' mindfulness in communication mitigates employees' knowledge hiding behavior. We propose a moderated model with two serial mediation paths, in which leaders' mindfulness in communication enhances employees' perceived organizational support, which subsequently reduces knowledge hiding through increased organizational justice and strengthened interpersonal trust, respectively. We further examine employee mindfulness as a boundary condition that amplifies the positive relationship between leaders' mindfulness in communication and perceived organizational support. Study 1 used a three-wave survey of 652 employees from Chinese technology firms to test the proposed model. Study 2 employed a four-week mindfulness training intervention involving 180 leaders and 540 employees to provide supplementary evidence for the intervention effect. The results showed that leaders' mindfulness in communication was negatively associated with employees' knowledge hiding through two pathways: perceived organizational support and organizational justice, and perceived organizational support and interpersonal trust. Employee mindfulness strengthened the relationship between leaders' mindfulness in communication and perceived organizational support. The intervention study further showed that training both leaders and employees led to greater reductions in knowledge hiding than training leaders alone or providing no mindfulness training. These findings extend research on knowledge hiding and workplace mindfulness by integrating exchange-based mechanisms with social information-processing perspectives, and they offer a practical intervention approach for promoting supportive communication and reducing defensive knowledge behaviors in organizations.