Background: It has been reported that coronary slow flow (CSF) is associated with acute myocardial infarction, ventricular tachycardia, ventricular fibrillation, and even sudden cardiac death. Although studies concerning the etiopathogenesis of CSF are scarce, diffuse atherosclerosis and endothelial dysfunction are thought to play important roles. It has been suggested that a high plasma thrombomodulin (TM) level seems to play an important role in the pathogenesis of atherosclerosis and endothelial dysfunction. Objectives: We hypothesized that a high plasma TM level might be associated with CSF and aimed to research the relationship between plasma TM level and CSF. Methods: Fifty-two CSF patients with angiographically proven CSF and 44 cases with normal coronary flow were included in this study. Coronary flow velocity was determined by the thrombolysis in myocardial infarction (TIMI) frame count method. Plasma TM levels were measured in all the study subjects. Results: Plasma TM levels were significantly higher in the CSF group compared to the control group (3.9 ± 0.5 vs. 3.6 ± 0.3 ng/mL, p = 0.01). There was a positive relationship (r = 0.31, p = 0.002) between plasma TM level and mean TIMI frame count (TFC). Factors associated with mean TFC were plasma TM level (β = 0.206, p = 0.038) and red cell distribution width (β = 0.088, p = 0.009) in multiple linear regression analysis. Conclusions: Patients with CSF have a higher plasma TM level, and this may play an important role in the pathogenesis of CSF. An elevated plasma TM level may be a predictor of CSF. Future studies are needed to confirm these results.