Background: Homeostasis of thyroid hormones has significant effects on the cardiovascular system. The aim of this study was to investigate the association between free triiodothyronine (FT3) and adverse cardiovascular events in patients with acute coronary syndrome (ACS) who were undergoing percutaneous coronary intervention (PCI). Methods: A total of 1701 patients with ACS undergoing PCI were included in this study. All patients were divided into three groups according to the tertiles of FT3 level: the lowest tertile (FT3 4.51 pmol/L), the middle tertile (4.51 pmol/L \le FT3 4.89 pmol/L) and the highest tertile group (FT3 \ge4.89 pmol/L). The primary study endpoint was a composite of major adverse cardiovascular events (MACE), which included all-cause death, ischemic stroke, myocardial infarction, or unplanned repeat revascularization. Results: During a median follow-up period of 927 days, 349 patients had at least one event. Compared with patients with the highest tertile, those with the lowest tertile had a significantly higher incidence of MACE, all-cause death, MI, ischemic stroke and repeat revascularization (all p values 0.05). In the multivariate Cox regression analysis, the middle tertile had similar risk of MACE (HR = 0.986, 95% CI 0.728–1.336, p = 0.929) as the highest tertile, but the patients with the lowest tertile had a 92.9% higher risk of MACE (HR = 1.929, 95% CI 1.467–2.535, p 0.001). There was a non-linear relationship between FT3 and MACE and unplanned repeat revascularization (all p values for non-linear association 0.001). Adding the tertiles of FT3 level into the baseline model yielded a significant improvement in discrimination for predicting MACE (\mathrm{\Delta }AUC = 0.013, p = 0.025). Conclusions: A significantly reduced FT3 level was independently associated with a worse prognosis in patients with ACS undergoing PCI.