Background: Obesity adversely affects health and is associated with subclinical systemic inflammation and features of accelerated aging, including the T-cell immune system. The presence of metabolic syndrome (MetS) may accelerate, while bariatric surgery might reverse these phenomena. To examine the effects of MetS and bariatric surgery on T-cell aging, we measured relative telomere length (RTL) and T-cell differentiation status in obese patients before and after bariatric surgery. Methods: WHO II/III classified obese patients scheduled for bariatric surgery were included: 41 without MetS and 67 with MetS. RTL and T-cell differentiation status were measured in circulating CD4 + and CD8 + T cells via flow cytometry. T-cell characteristics were compared between patients with and without MetS prior to and at 3, 6, and 12 months after surgery considering effects of age, cytomegalovirus-serostatus, and weight loss. Results: Thymic output, represented by numbers of CD31-expressing naive T cells, showed an age-related decline in patients with MetS. MetS significantly enhanced CD8 + T-cell differentiation. Patients with MetS had significant lower CD4 + RTL than patients without MetS. Within the first 6 months after bariatric surgery, RTL increased in CD4 + T cells after which it decreased at month 12. A decline in both thymic output and more differentiated T cells was seen following bariatric surgery, more pronounced in the MetS group and showing an association with percentage of body weight loss. Conclusions: In obese patients, MetS results in attrition of RTL and accelerated T-cell differentiation. Bariatric surgery temporarily reverses these effects. These data suggest that MetS is a risk factor for accelerated aging of T cells and that MetS should be a more prominent factor in the decision making for eligibility for bariatric surgery.