Background and Objectives: Type 2 diabetes (T2D) is an independent risk factor for the development of atrial fibrillation (AF). Sodium-glucose cotransporter-2 inhibitors (SGLT-2i) have recently been shown to decrease the incidence of AF through several mechanisms, including the reduction of atrial dilatation via diuresis and the lowering of body weight. In observational studies of diabetic patients, the use of thiazolidinedione (TZD) was found to have a protective effect on new-onset AF. In this study, we aimed to compare the effect of SGLT-2i and TZD on the risk of AF in patients with T2D. Methods: We enrolled 69,122 patients newly prescribed SGLT-2i and 94,262 patients prescribed TZD from January 2014 to December 2018, using the Korean National Health Insurance Service database. We compared new-onset AF events (hospitalizations and outpatient events) in SGLT-2i and TZD groups after having taken medication for greater than 90 days. Results: During a mean follow-up of 1.8 years, 397 (0.72%) new-onset AF events occurred in the SGLT-2i group and 432 (0.79%) events in the TZD group following propensity score matching (each group n = 54,993). The hazard ratio (HR) of AF was 0.918 (95% confidence interval: 0.783–1.076, p = 0.29) in SGLT-2i-treated patients compared with TZD-treated patients. Conclusions: In this study, the risk of new-onset AF is comparable in patients treated with SGLT-2i and TZD in T2D. Either SGLT-2i or TZD would be a reasonable choice for T2D patients who are at risk for AF.