Background and Aims: Malnutrition is a condition that has a great impact on oncology patients. Poor nutritional status is often associated with increased morbidity and mortality, increased toxicity, and reduced tolerance to chemotherapy, among other complications. The recently developed GLIM criteria for malnutrition aim to homogenize its diagnosis, considering the baseline disease status. We aimed to evaluate the performance of these new criteria for the prediction of complications and mortality in patients with cancer. Methods: This work is a prospective, single-center study. All outpatients under active treatment for head and neck, upper gastrointestinal, and colorectal tumors between February and October 2020 were recruited. These patients were followed up for 6 months, assessing the occurrence of complications and survival based on GLIM diagnoses of malnutrition. Results: We enrolled 165 outpatients, 46.66% of whom were malnourished. During the 6-month follow-ups, patients with malnutrition (46.7%, according to GLIM criteria) had a ~3-fold increased risk of hospital admission (p < 0.001) and occurrence of severe infection (considered as those requiring hospitalization, intravenous antibiotics, and/or drainage by interventional procedures) (p = 0.002). Similarly, malnourished patients had a 3.5-fold increased risk of poor pain control and a 4.4-fold increased need for higher doses of opioids (both p < 0.001). They also had a 2.6-fold increased risk of toxicity (p = 0.044) and a 2.5-fold increased likelihood of needing a dose decrease or discontinuation of cancer treatment (p = 0.011). The 6-month survival of malnourished patients was significantly lower (p = 0.023) than in non-malnourished patients. Conclusions: Diagnoses of malnutrition according to the GLIM criteria in oncology patients undergoing active treatment predict increased complications and worse survival at 6-month follow-ups, making them a useful tool for assessing the nutritional status of oncology patients.