To investigate the prognostic significance of hospitalization-associated muscle weakness for short-term and long-term functional declines among oldest old hospitalized patients. We conducted a retrospective cohort study in the Geriatric Evaluation and Management Unit (GEMU) of a tertiary medical center in Taiwan. Data from patients admitted to the GEMU were collected, including demographic characteristics, underlying diseases, functional assessments, and nutritional status. Handgrip strength was measured at admission and before discharge, and hospitalization-associated muscle weakness was defined accordingly. Functional status was ascertained by a telephone survey, and functional decline was defined based on a comparison with each patient's functional status before discharge. Overall, data from 555 patients were retrieved, and 431 patients (mean age: 86.0±6.2years, 78.5% males) were included in the study, with a mean Barthel Index score of 74.7±20.8, Mini-Mental State Examination (MMSE) score of 19.1±6.8, Geriatric Depression Scale (GDS-5) score of 2.2±1.6, Mini-Nutritional Assessment-Short Form (MNA-SF) score of 8.9±3.0, Cumulative Illness Rating Scale-Geriatric (CIRS-G) score of 2.2±0.9, and the regular use of 6.7±3.5 medications. Patients with a 1-month postdischarge functional decline had significantly lower body weight and body mass index, more severe frailty and more hospitalization-associated muscle weakness. Patients with a 6-month postdischarge functional decline had lower body weight, BMI, MMSE scores, MNA-SF scores, handgrip strength, Barthel Index scores, and Braden Scale scores. BMI (OR: 0.815, 95% CI: 0.691-0.962, P=0.015) and hospitalization-associated muscle weakness (OR: 1.227, 95% CI: 1.147-1.332, P<0.001) were significantly associated with a 1-month postdischarge functional decline. Hospitalization-associated muscle weakness (OR: 1.067, 95% CI: 1.035-1.101, P<0.001), the risk of pressure sores (Braden Scale, OR: 0.767, 95% CI: 0.646-0.910, P=0.002), and BMI (OR: 0.914, 95% CI: 0.838-0.998, P=0.045) were independently associated with a 6-month postdischarge functional decline. Hospitalization-associated muscle weakness significantly predicts functional decline among oldest old patients hospitalized for acute conditions. Further interventional studies are needed to examine the causal relationship between hospitalization-associated muscle weakness and clinical outcomes.