Background/Objectives: Malnutrition is a common issue among cancer patients, leading to adverse outcomes and reduced survival rates; therefore, nutritional screening and assessment are essential. Our study aims to evaluate the diagnostic efficacy of the GLIM criteria to assess malnutrition and to compare the performance of various nutritional screening tests in evaluating nutritional risk. Methods: A cross-sectional observational study was conducted on outpatient oncologic patients. Malnutrition was assessed using PG-SGA and GLIM criteria. Nutritional screening was performed using NRS-2002, MUST, MST, and NUTRISCORE. Anthropometric measurements, handgrip strength, cancer medical records and descriptive data were collected and analyzed. Results: A total of 396 patients were involved. Malnutrition was diagnosed in 37.4% according to GLIM criteria and in 25.8% according to PG-SGA (p<0.001). The agreement between GLIM criteria and PG-SGA was substantial (ƙ= 0.63). The diagnostic performance of PG-SGA showed a sensitivity of 93% and a specificity of 96.3%, whereas the GLIM criteria demonstrated a sensitivity of 81.3% and a specificity of 91.2%. Both tools had a significant association with low dynamometry and low BMI (p<0.001). Nutritional risk was identified in 22.7% of the patients using NRS-2002, 25.8% with MUST, 26.5% with MST, and 12.9% with NUTRISCORE. The highest agreement among the nutritional screening tests was observed between MUST and PG-SGA (ƙ =0.64). Conclusions: GLIM criteria detected a higher prevalence of malnutrition compared to PG-SGA; however, PG-SGA demonstrated higher sensitivity and specificity. Both significantly agree with low dynamometry and low BMI. Among the nutritional screening tests, MUST and PG-SGA showed the highest agreement.