Background <p>The predictive ability of vitamin D for weight loss after bariatric surgery is not well-characterized. This prospective cohort study assessed the utility of preoperative serum 25-hydroxyvitamin D (25(OH)D) as a predictor of body mass index (BMI) at follow-up after bariatric surgery. Additionally, the mediation role of serum albumin was explored.</p> Methods <p>Patients scheduled for bariatric surgery were recruited from a single center. The final analyzed cohort consisted of 316 subjects (age, 40 ± 10 years; 35.4% males; BMI, 42.6 ± 7.2 kg/m²). The associations between preoperative 25(OH)D and baseline (pre-surgery) BMI or repeated measures of BMI collected at baseline, and 3-, 6-, and 12-month post-surgery were examined using linear regression or linear mixed model, respectively. The mediation effect of baseline albumin was evaluated using mediation analysis.</p> Results <p>Before surgery, 98.5% of patients had vitamin D insufficiency (25(OH)D &lt; 30 μg/L). Baseline BMI elevated progressively as severity of vitamin D insufficiency increased (<i>P</i>-trend = 0.025). Lower 25(OH)D levels were independently associated with higher preoperative BMI (coefficient, − 0.20; 95% CI, − 0.32 to − 0.08; <i>P</i> = 0.001) or less BMI reduction at follow-up (coefficient, − 0.15; 95% CI, − 0.25 to − 0.04; <i>P</i> = 0.007), after adjustment for baseline demographics, diabetes status, and/or surgical procedure. The association diminished after accounting for albumin, which emerged as a significant determinant (coefficient, − 0.61; 95% CI, − 0.83 to − 0.40; <i>P</i> &lt; 0.001). Mediation analysis showed that reduced albumin explained 30% (<i>P</i> &lt; 0.001) of the relationship between 25(OH)D and longitudinal BMI.</p> Conclusions <p>Lower preoperative 25(OH)D is associated with less BMI reduction over 1-year follow-up after bariatric surgery, potentially mediated by reduced serum albumin.</p> Graphical Abstract <p></p>