Red cell distribution width (RDW) can effectively predict prognosis in coronary artery disease (CAD) patients following percutaneous coronary intervention (PCI). There is currently no relevant research to demonstrate a linear or non-linear association between RDW and mortality. This is a multi-center, retrospective cohort study, with data collected from 2006 to 2017. Source data included electronic medical records of the Integrated Medical Database of National Taiwan University Hospital, and health insurance claims from the National Health Insurance Administration. Patients were stratified into five groups according to RDW values (13.4%, 14.1%, 14.8%, and 15.9%). Multivariable logistic and Cox regression analyses were used to determine 1-year all-cause and cardiovascular (CV) mortalities. Data of 10,669 patients were analyzed and those with the lowest RDW (≤13.3%) served as the reference group. The adjusted odds ratios (ORs) of 1-year all-cause mortality from the second to fifth RDW group were 1.386, 1.589, 2.090, and 3.192, respectively (p for trend < 0.001). The adjusted ORs of 1-year CV mortality were 1.555, 1.585, 1.623, and 2.850, respectively (p for trend = 0.015). The adjusted hazard ratios (HRs) of 1-year all-cause mortality were 1.394, 1.592, 2.003, and 2.689, respectively (p for trend = 0.006). The adjusted HRs of 1-year CV mortality were 1.533, 1.568, 1.609, and 2.710, respectively (p for trend = 0.015). RDW was an independent predicting factor and had a linear relationship with the 1-year all-cause and CV mortalities in patients undergoing PCI. Thus, RDW may be a clinically useful parameter to predict the mortality in those patients.