Background and Objectives: Lymphocyte-to-monocyte ratio (LMR) has emerged as a new indirect marker of inflammation, which is associated with adverse outcomes in cardiovascular diseases. The aim of this study was to evaluate whether admission LMR is associated with contrast-induced nephropathy (CIN) in patients who underwent percutaneous coronary intervention for acute coronary syndrome (ACS). Methods: A total of 873 patients were assessed. LMR was calculated via dividing lymphocyte count by monocyte count. Results: LMR was significantly lower in the with-CIN group. ROC analysis showed that the LMR ratios <2.52 predicted CIN development with sensitivity of 66.3% and specificity of 55.8%. Multivariate analysis showed that eGFR, admission glucose, and LMR were independent predictors of CIN in patients with ACS. Conclusion: LMR is an easily accessible marker and could be used as a predictor of CIN in patients with ACS undergoing percutaneous coronary intervention.