Introduction Cancer-related malnutrition is a highly prevalent, yet often overlooked concern in clinical practice. Although cancer-related management guidelines recommend standardized nutritional care, its implementation is scarce. The aim of this study was to investigate the prevalence of malnutrition and the medical need for nutrition counseling in cancer patients employing a novel standardized nutritional management program (containing malnutrition risk screening, nutritional assessment and counseling). Furthermore, differences of malnutrition parameters in different cancer patient cohorts were examined. Methods Cancer patients were screened for malnutrition using the Patient-Generated Subjective Global Assessment Short Form (PG-SGA SF) on the first day of their inpatient admission to the internal oncology or hematology wards. PG-SGA total score and classification into the three PG-SGA nutrition stages (A, B, C) were used to determine nutritional status. In case of a positive screening, nutritional assessment and individualized counseling by a nutritionist followed. For group comparisons, patients were divided into different groups (e.g., age, gender, tumor entity) and were evaluated accordingly. Results A total of 1,100 inpatients were included. 56.8 % of the patients had suspected or already existing malnutrition. The most common nutrition impact symptom was loss of appetite (26.7 %), followed by fatigue (16.5 %) and pain (16.0 %). Female (p < 0.001), elderly (p < 0.001) and patients with upper gastrointestinal tract tumors (p < 0.001) showed an unfavorable nutritional status and higher need for counseling. Despite suffering from malnutrition, patients had Body Mass Indices within the upper end of the normal range. Conclusion This study shows a high prevalence of malnutrition in hospitalized cancer patients and highlights the need for a standardized nutritional management in the clinical setting. Therefore, it is recommended to provide a malnutrition risk screening for all cancer patients and a following adequate assessment and personalized nutritional care if needed.