Objective To investigate the hypothesis that free light chain (FLC) sera levels could serve as a biomarker for myasthenia gravis (MG), especially for the subgroups of seronegative MG and ocular MG. Methods Sera from 73 patients with MG (20 seronegative for antiacetylcholine receptor [AChR] and anti–muscle-specific kinase and 53 positive for anti-AChR, which were clinically divided into 24 patients with ocular type, 45 with generalized type, and 4 with unequivocal clinical manifestation) and 49 healthy controls were studied for κ FLC and λ FLC levels with the Freelite human FLC kits. Results The κ but not the λ levels of FLC were significantly increased in the patients with MG, including those with double seronegative MG and ocular MG, compared with the healthy controls. The specificity for double seronegative MG and ocular MG were both 98.0% when κ FLC was ≥25.0 mg/L. Increased κ FLC levels were not affected by the patient’s sex, age at MG onset, the presence of thymic pathology, or different treatments. Conclusions Elevated serum κ FLC may serve as a biomarker for MG in suspected patients who are double seronegative and in those with only ocular manifestations when serology is inconclusive. Classification of evidence This study provides Class III evidence that high κ FLC levels distinguished patients with MG, including those who were double seronegative, from healthy controls.