Background: The detection and management of dehydration represent critical challenges within the older adult population. Although the factors influencing dehydration have been studied previously, its psychological impacts have mostly been examined in younger populations. This study investigated the associated factors for dehydration and its relationship with depression in individuals aged over 80 years. Methods: This retrospective cross-sectional study included 263 individuals aged 80 and above who visited the Healthy Aging Center of a tertiary hospital between March and September 2024. Data were obtained from medical records, including sociodemographic information, comorbidities, and scores from validated geriatric assessment tools: Clinical Frailty Scale (CFS), Katz Index of Independence in Activities of Daily Living (ADL), Lawton-Brody Instrumental Activities of Daily Living Scale (IADL), Mini Nutritional Assessment—Short Form (MNA-SF), and Geriatric Depression Scale (GDS). Dehydration was defined as a calculated plasma osmolality greater than 295 mOsm/L. Results: Dehydration was present in 68.4% of participants. Statistically significant associations were found between dehydration and higher GDS scores, frailty status, lower ADL and IADL scores, higher TSH levels, and the presence of diabetes mellitus (DM) and chronic kidney disease (CKD). Conclusions: A significant association was observed between dehydration and depression. Regression analysis identified GDS score as an independent factor associated with dehydration.