Background/Objectives: Malnutrition and nutritional vulnerability are common in patients with cancer and are associated with adverse clinical outcomes, particularly among hospitalised patients. However, data specifically describing nutritional risk in patients admitted to medical oncology departments remain limited. This study aimed to estimate the prevalence of nutritional risk at hospital admission and to evaluate factors associated with nutritional risk and its clinical consequences. Methods: The REGIO registry is a national multicentre prospective cohort including adult patients with solid tumours admitted to medical oncology departments in 17 Spanish hospitals between February 2024 and January 2025. Nutritional risk was assessed within the first working day of hospitalisation using the Malnutrition Screening Tool (MST), with a score ≥ 2 indicating nutritional risk. Multivariable logistic regression models were used to identify factors associated with nutritional risk and its association with prolonged hospital stay and mortality. Results: A total of 1229 patients were included (median age 67.8 years; 59% male; 64% metastatic disease). Nutritional risk was identified in 53% of patients. In multivariable analysis, poorer functional status, tumour progression, recent exposure to cytotoxic chemotherapy, and tumour types with higher nutritional impact were independently associated with nutritional risk. Patients at nutritional risk had longer hospital stays (median 10 vs. 7 days; p < 0.001), a higher likelihood of prolonged hospitalisation (adjusted OR 1.38), and increased mortality at 30 days (adjusted OR 1.63) and 60 days after discharge (adjusted OR 1.53). Conclusions: In this large multicentre cohort, nutritional risk was highly prevalent and independently associated with worse clinical outcomes, supporting the clinical relevance of systematic nutritional screening at hospital admission in patients with cancer.