Background: Coronary Slow-Flow Phenomenon (CSFP) poses a diagnostic challenge in the context of coronary artery disease (CAD), and the search for reliable biomarkers continues. The Triglyceride/High-density Lipoprotein Cholesterol (TG/HDL-C) ratio has emerged as a potential candidate, given its established role in atherogenesis. This study aimed to investigate the relationship between the TG/HDL-C ratio and CSFP, assessing its potential as a diagnostic marker. Methods: This prospective interventional study included 100 patients who underwent elective coronary angiography for suspected CAD. The patients was divided into two groups, 50 patients has CSFP and 50 controls. On admission to the ER, all patients were subjected to physical examination including vital data and local examination, and 12 lead ECG, laboratory investigations including, cardiac enzyme, lipid profile random blood sugar, complete blood picture and kidney function tests. Results: The study revealed a significantly higher TG/HDL-C ratio in the CSFP group compared to controls (p < 0.001). ROC analysis indicated the TG/HDL-C ratio as a moderately accurate diagnostic tool for CSFP (AUC = 0.822). Significant correlations were observed between the TG/HDL-C ratio and key parameters, including non-HDL-C, triglycerides, ECG features (P max, PWd, and QTcd), while negatively correlating with HDL-C, WBC, and QTc. Logistic regression affirmed the TG/HDL-C ratio as the sole parameter associated with CSFP risk. Conclusion: Among patients with suspected CAD, the TG/HDL-C ratio strongly was associated with CSFP and serves as a noteworthy independent predictor of CSFP.