Abstract Objective Although malnutrition is common in cancer patients in Korea, little attention is paid to its risks and consequences. This study was carried out to investigate the prevalence and risk factors of malnutrition in hospitalized cancer patients according to tumor location and stage. Methods Of 14 972 cancer patients admitted to the National Cancer Center, screening examinations were carried out for 12 112 patients and nutritional status was assessed in 8895 patients. Information on age, sex, length of hospital stay, and tumor location and stage were collected from the electronic medical records system. The nutritional status of each subject was assessed using body mass index, serum albumin, total lymphocyte count, and diet and classified into three groups: high risk, moderate risk, and low risk of malnutrition. Results About 61% of hospitalized patients were malnourished and the prevalence of malnutrition was higher in male patients with longer hospital stays (60.2%, P = 0.0101) and readmitted patients (66.6%, P < 0.0001). Patients with liver and lung cancer (86.6% and 60.5%, respectively) and patients with advanced cancer stage (60.5%, III or IV) had a higher prevalence of malnutrition than other patients ( P < 0.0001). Logistic regression analysis showed that patients with advanced cancer stage and longer hospital stay and readmitted patients were at a higher risk for malnutrition. Conclusion The prevalence of malnutrition in hospitalized cancer patients was high and varied across tumor location and stage. Early identification of malnutrition status is required for proper nutritional intervention during hospitalization.