Background and objectives: No-reflow (NR) phenomenon is defined as insufficient myocardial perfusion in coronary circulation in the absence of angiographic evidence of mechanical obstruction. The primary mechanisms of the NR occurrence are thought to be high platelet activity and thrombus burden. Soluble CD40 ligand (sCD40L), which is released into the plasma following platelet activation, accelerates the inflammatory process and causes further platelet activation. The aim of our study is to investigate the relationship between the NR phenomenon and sCD40L level in patients with ST-elevation myocardial infarction (STEMI). Methods: A total of 81 acute STEMI patients undergoing primary percutaneous coronary intervention and 40 healthy participants were included in this study. Acute STEMI patients were classified into two groups: 41 patients with the NR phenomenon (NR group) and 40 patients without the NR phenomenon (non-NR group). The serum sCD40L level was measured for all groups. Results: The serum sCD40L level was significantly higher in the NR group than in non-NR and control groups (379 ± 20 pg/mL, 200 ± 15 pg/mL and 108 ± 6.53 pg/mL, respectively; p < 0.001). Univariate regression analysis demonstrated that male sex, age, Gensini score and sCD40L level were the possible factors affecting the occurrence of the NR phenomenon. In multivariate regression analysis, age (odds ratio [OR], 1.091; 95% confidence interval [CI], 1.023–1.163; p < 0.008) and serum sCD40L (OR, 1.016; 95% CI, 1.008–1.024; p < 0.001) remained the independent predictor of the presence of NR. Conclusions: Our study showed that serum sCD40L level was an independent predictor of the NR phenomenon occurrence.