Purpose@#To compare serum selenium levels in Graves patients and non-Graves control participants and to evaluate associations between serum selenium levels and clinical features of Graves orbitopathy (GO). @*Methods@#We conducted a single-center, retrospective case-control study among 33 patients with Graves disease without GO (GD), 31 patients with diagnosed GO, and 27 unaffected healthy participants enrolled between 2013 and 2020 at Severance Hospital. We compared serum selenium concentrations between the GD, GO, and healthy control groups, and analyzed associations between serum selenium and GO patients’ clinical activity scores, severity (assessed through modified NOSPECS scores), and other clinical features using multivariate linear regression analysis. @*Results@#Mean serum selenium levels were 109.30 ± 16.39, 111.39 ± 14.04, and 126.09 ± 21.09 ng/mL in GO patients, GD patients, and healthy control participants, respectively. Mean serum selenium levels in Graves patients with and without orbitopathy were significantly lower than those in the healthy control group (p < 0.05), and mean selenium levels were slightly lower in GO than those in GD patients (p = 0.594). Serum selenium levels were significantly lower in GO patients with eyelid retraction than in patients without retraction (p = 0.038). However, serum selenium levels were not associated with clinical activity scores and modified NOSPECS scores (p = 0.241 and 0.801, respectively). @*Conclusions@#Serum selenium levels were significantly lower in Graves patients with or without GO, compared to non-Graves control participants. Selenium levels were not associated with clinical activity scores or NOSPECS scores, though we observed an association with eyelid retraction.