Adequate hydration is essential for health, with even mild forms of dehydration often having negative effects on cognition and well-being. Despite evidence of higher risk for dehydration among older adults, links between dehydration and cognitive or well-being outcomes have not been established in old age. In this study, we used longitudinal data from the Berlin Aging Study II (age range 60-89) to investigate whether trajectories of cognitive functioning (digit symbol, N = 1,111) and well-being (Diener satisfaction with life, N = 1,066; Socio-Economic Panel Study life satisfaction, N = 1,067; and Lawton morale, N = 1,067) are associated with objective dehydration (osmolarity; 33% dehydrated). Our results revealed that higher dehydration was associated with steeper decline in cognitive functioning and well-being over time, and lower well-being among those with higher body mass index. These associations were independent of sociodemographic and physical health characteristics. Our findings highlight the importance of adequate hydration for preserved cognition and well-being across old age. We discuss potential mechanisms and consider practical implications arising from our results. (PsycInfo Database Record (c) 2020 APA, all rights reserved).