Background: Thymic Carcinoma is a rare malignancy with an aggressive clinical course. While the importance of surgery in the non-metastatic setting has been well defined, the optimal role of radiation and/or systemic therapy in this setting remains controversial. This study utilized the Surveillance, Epidemiology, and End Results (SEER) database to investigate the impact of adjuvant therapy on overall survival in patients with thymic carcinoma. Methods: We identified adults in the SEER database with thymic carcinoma diagnosed between 1989 to 2015 for analysis. As the primary treatment for non-metastatic thymic carcinoma is surgery, we excluded patients who did not have surgery as a component of their treatment Patients were categorized into Masaoka-Koga stage groups (I-IIa, IIb, III, and IV). Kaplan-Meier estimates of 10-year OS and multivariate Cox proportional hazards regression analyses were performed. Results: 515 patients met the inclusion criteria, of which 125 were stage I-IIa, 46 were stage IIb, 191 were stage III, and 143 were stage IV. A multivariate analysis was performed, adjusting for age, sex, race, and tumor size. When compared to surgery, no statistical improvement in survival was seen with adjuvant radiation or chemotherapy in stage I-IIa or IIb thymic carcinoma. In stage III disease, standard of care surgery was compared with adjuvant radiation (hazard ratio 0.69 (95% confidence interval 0.29 – 1.63], p = 0.39), adjuvant chemotherapy (hazard ratio [HR] 0.84 [95% confidence interval 0.28 – 2.53], p = 0.76), and adjuvant chemo-radiotherapy (HR 0.40 [0.18 – 0.93], p = 0.03). On Kaplan-Meier analysis, triple therapy (surgery + chemo-radiotherapy) was also associated with a marked improvement in 10-year overall survival at 55.3%, compared to 39.8%, 23.9%, and 20.8% (adjuvant radiation, adjuvant chemotherapy, and surgery alone, respectively). Conclusions: This study finds that in stage III thymic carcinoma, surgery followed by chemo-radiotherapy is associated with improved overall survival compared to single or dual-modality treatments. Because of the rarity of this disease, there are no large randomized studies evaluating the most appropriate treatment modalities, therefore this data may assist with clinical decision making in non-metastatic thymic carcinoma.