Objective: This article focuses on retirement transition from the Conservation of Resources (COR) perspective to better understand how aged participants’ perceptions of retirement losses and gains significantly explain retirement well-being. In this article, the mediation of social support in the losses–well-being relationship is explored. Method: The study was conducted with a two-wave longitudinal design. Participants at T1 were aged Spanish workers (>64 years) and at T2 were retirees, with a final sample of 275, who had retired during the previous 6 months. Results: Findings supported the assertion that losses better explain well-being than gains. In addition, specific losses revealed a higher explaining power of life satisfaction and health complaints depending on their content. Social support mediated between perceived losses and well-being. Discussion: This study suggests that both perceived losses and gains associated with retirement and social support during retirement should be taken into account when addressing postretirement well-being.