(1) Background: Bariatric surgery has demonstrated the capacity to improve metabolic-associated fatty liver disease (MAFLD) in patients with morbid obesity. In addition, the Mediterranean diet contains anti-inflammatory, anti-oxidative, and anti-fibrotic components, promoting a beneficial effect on MAFLD. This study aimed to assess the improvement of MAFLD, specifically liver steatosis, in morbidly obese patients undergoing Roux-en-Y gastric bypass (RYGB) and following a hypocaloric Mediterranean-like diet. (2) Methods: A prospective observational pilot study of 20 patients undergoing RYGB was conducted. The participants underwent a magnetic resonance spectroscopy study 2 weeks before the surgical act and one year postoperatively to assess the percentage of lipid content (PLC). The adherence to the Mediterranean diet was determined by the KIDMED test 1 year after surgery. (3) Results: Mean baseline PLC was 14.2 ± 9.4%, and one year after surgery, it decreased to 4.0 ± 1.8% (p < 0.001). A total of 12 patients (60%) were within the range of moderate adherence to the Mediterranean diet, whereas 8 patients (40%) showed a high adherence. The patients with high adherence to the Mediterranean diet presented significantly lower values of postoperative PLC. (4) Conclusions: Liver steatosis significantly reduces after RYGB. This reduction is further improved when associated with a high adherence to a Mediterranean diet.