Abstract: Objective To investigate structural differences in the hypothalamus between obese patients and healthy volunteers using multimodal magnetic resonance imaging (MRI), and to further analyze the correlations between hypothalamic structural characteristics and postoperative weight changes following bariatric surgery. Methods A total of 52 obese patients scheduled for bariatric surgery and 45 healthy volunteers recruited between September 2023 and June 2024 were enrolled. All participants underwent cranial MRI scans. Clinical data, questionnaire scores, hypothalamic and subregional volumes, and microstructural parameters were compared between the two groups. Six months after surgery, changes in body weight, waist circumference, hip circumference, and neck circumference were recorded. The percentage of total weight loss (%TWL) and excess weight loss (%EWL) were calculated. Correlations between hypothalamic structural indicators and obesity indices, questionnaire scores, and weight loss outcomes were analyzed. Results Compared with healthy volunteers, obese patients showed significantly increased volumes in the left anterior-inferior and right posterior regions of the hypothalamus, as well as significantly elevated mean diffusivity (MD) values. The volume and MD values of the left anterior-inferior hypothalamus were significantly positively correlated with body weight, BMI, waist circumference, hip circumference, and neck circumference (P<0.01). The hypothalamic MD value was also positively correlated with the emotional eating subscale score of the Dutch Eating Behavior Questionnaire (r=0.272, P=0.007). No significant correlations were found between hypothalamic abnormalities or MD values and postoperative weight loss indicators at postoperative 6 months. Conclusions Obese individuals exhibit increased volume and elevated MD values in the left anterior-inferior and right posterior hypothalamic regions, both significantly correlated with obesity-related anthropometric measures. These findings suggest that the hypothalamus plays an important role in the onset and progression of obesity.