Background and Objectives: Adverse clinical outcomes are associated with malnutrition and frailty, which are highly prevalent among hospitalized older patients. This study aimed to evaluate their predictive value for the duration of hospitalization, short-term survival, and rehospitalization of patients admitted to internal medicine wards. Materials and Methods: This prospective cohort study included 134 acutely ill patients aged ≥50 years who were hospitalized in an internal medicine department and evaluated within the first 48 h of admission. Nutritional status was evaluated using the Mini nutritional assessment–short form (MNA-SF), Nutritional Risk Screening 2002 (NRS-2002), and Global Leadership Initiative on Malnutrition (GLIM) criteria. Frailty was evaluated using the FRAIL scale and Clinical Frailty Scale (CFS). The primary outcomes were prolonged hospitalization (>10 days), mortality, and rehospitalization at 3 and 6 months post-discharge. Results: According to MNA-SF, 33.6% of patients were malnourished; 44% had nutritional risk per NRS-2002, and 44.8% were malnourished per GLIM. Frailty prevalence was 53.7% (FRAIL) and 59% (CFS). Malnutrition defined by all three scales (MNA-SF, NRS-2002, GLIM) was significantly associated with prolonged hospitalization (p = 0.043, 0.014, and 0.023, respectively), increased rehospitalization at both 3 months (p < 0.001) and 6 months (p < 0.001). Mortality was also significantly higher among malnourished patients. Higher CFS scores and low handgrip strength were additional predictors of adverse outcomes (p < 0.05). In multivariable analysis, GLIM-defined malnutrition and CFS remained independent predictors of rehospitalization and mortality. Conclusions: Frailty and malnutrition are highly prevalent and independently associated with prolonged hospital stay, short-term rehospitalization and mortality. Routine screening at admission may facilitate early identification and guide timely interventions to improve patient outcomes. These findings might guide hospital protocols in aging health systems and support the development of standardized geriatric care pathways.