Background: Malnutrition is common in patients undergoing hemodialysis and leads to a decrease in dialysis adequacy. The evaluation of dialysis adequacy is very important. Objectives: This study was done to investigate the relationship between patients’ nutrition status based on Subjective Global Assessment (SGA) and their dialysis adequacy. Methods: This cross-sectional descriptive study was conducted in Tehran, Iran, in 2019. Three hundred qualified patients were selected by cluster sampling. Before the dialysis session, measurement of height and weight and blood sampling for albumin, total iron-binding capacity (TIBC), blood urea nitrogen (BUN), and creatinine (Cr) assessment were performed. After the dialysis session, measurement of weight and second sampling for BUN were done. SGA forms were filled by patients. Patients’ dialysis adequacy was calculated based on the DaugirdasII formula, and data were analyzed using SPSS 21. Results: From 300 patients, 128 cases (42.7%) had normal nutrition status, while 148 cases (49.3%) had mild to moderate malnutrition and 24 cases (8%) had severe malnutrition with mean dialysis adequacy of 1.07. The present study showed a significant statistical relationship between malnutrition and education (P < 0.001), body mass index (BMI) (P = 0.03), albumin (P < 0.001), TIBC (P < 0.001), and dialysis adequacy (P < 0.001). Gender, age, and dialysis duration showed no significant relationship with nutrition status based on the SGA index. Conclusions: The findings showed a high relative frequency of malnutrition and a decrease in patients’ dialysis adequacy, which emphasize the patient’s educational level, proper management of nutrition along with regular consultations by nutritionists, better implementation of the dialysis procedure, and regular follow-up of dialysis adequacy.