Perceived organizational support (POS) is critical for fostering discretionary service behaviors, particularly in demanding contexts such as luxury hospitality. This study examines how POS influences extra-role behavior (ERB) among frontline hotel employees by testing the mediating roles of job embeddedness (JEM), leader–member exchange (LMX), and work engagement (WEN), along with the moderating role of transformational leadership (TFL). Drawing on Conservation of Resources (COR) theory, survey data were collected from 170 full-time employees in five-star luxury hotels in Bangkok, Thailand, and analyzed using PLS-SEM. The results indicate that POS significantly predicts JEM, LMX, and WEN, and that each of these mechanisms, in turn, positively affects ERB. Moderation analysis further shows that TFL weakens the JEM–ERB link but strengthens the LMX–ERB relationship, while its interaction with WEN is nonsignificant. These findings suggest that structural mechanisms (POS and JEM) and relational quality (LMX) exert stronger effects than affective engagement in shaping discretionary service contributions. The study supports COR theory by showing that resources such as POS, JEM, and LMX foster discretionary service behavior. However, these effects are context-dependent, with transformational leadership serving as a boundary condition that may either enhance or diminish the translation of resources into extra-role contributions.