Background: Associations between D-dimer and outcomes of patients with acute coronary syndrome (ACS) remain controversial. Using age-adjusted D-dimer cutoff thresholds improve the diagnostic accuracy for thrombotic diseases. This study aimed to investigate the prognostic value of age-adjusted D-dimer in ACS patients treated by percutaneous coronary intervention (PCI). Methods: A total of 3972 consecutive patients with ACS treated by PCI were retrospectively recruited. The basal age-adjusted D-dimer threshold was 500 ng/mL and was calculated as age × 10 in patients older than 50 years. Cox regression was used for outcome analysis. C-index, net reclassification index (NRI), and integrated discrimination improvement (IDI) were calculated to assess the additional prognostic value of age-adjusted D-dimer when combined with established clinical risk factors. The primary outcome was all-cause death. Results: During a median follow-up of 720 days, a total of 225 deaths occurred. High D-dimer level, as defined by age-adjusted thresholds, was an independent predictor for all-cause death (hazard ratio [