Coronavirus disease (COVID-19) is a critical and potentially fatal condition. The nutritional status affects the evolution and clinical outcome throughout the disease course among factors influencing the patient prognosis. In patients with cancer, the Patient-Generated Subjective Global Assessment (PG-SGA) is the preferred instrument for assessing the nutritional status. The aim of this retrospective cross-sectional study was to evaluate the impact of the nutritional status on the clinical outcome of patients with COVID-19 undergoing cancer treatment. We enrolled 52 patients with cancer under outpatient follow-up who had been diagnosed with COVID-19 during the cancer treatment course. The PG-SGA instrument revealed age (p = 0.045) and nutritional status (p = 0.042) before infection as the main risk factors of death from COVID-19. In addition, the risk of mortality due to COVID-19 increased with the degree of malnutrition. Twelve (23.1%) of the 52 patients showed no negative effects related to COVID-19, and age below 65 years was considered to be a protective factor.