This study aims to identify the most accurate and useful non-invasive method to replace liver biopsy for the diagnosis of non-alcoholic fatty liver disease (NAFLD) before bariatric surgery and postoperative follow-up in morbidly obese patients. This single-center study is a retrospective analysis of prospectively collected data from 68 morbidly obese patients who underwent laparoscopic sleeve gastrectomy with intraoperative liver biopsy. Preoperative non-invasive diagnostic methods, including fatty liver index, NAFLD fibrosis score, enhanced liver fibrosis score, FibroScan, magnetic resonance imaging-proton density fat fraction (MRI-PDFF), magnetic resonance spectroscopy (MRS)-PDFF, and magnetic resonance elastography (MRE) were compared against liver biopsy results. Diagnostic performance was assessed using Spearman's correlation and receiver operating characteristic (ROC) curve analysis. Liver biopsy confirmed the presence of steatosis in 92.7% of patients, Nonalcoholic Steatohepatitis (NASH) in 64.7%, and liver fibrosis (≥F1) in 72.0%. MRI-PDFF and MRS-PDFF demonstrated the highest diagnostic accuracy for NASH, with the strongest correlation with histological findings. For liver fibrosis, MRE showed the strongest correlation with histological fibrosis stage, while FibroScan-Liver Stiffness Measurement (LSM) demonstrated better diagnostic performance in ROC analysis. However, the overall diagnostic quality of non-invasive methods for fibrosis assessment remained modest, with no method achieving a quality value above 0.6. MRI-PDFF and MRS-PDFF were the most accurate noninvasive methods for diagnosing NASH in morbidly obese patients. For liver fibrosis, FibroScan-LSM may be more suitable for detection, while MRE may better reflect fibrosis severity. Further studies are needed to assess the cost-effectiveness and clinical applicability of these methods.