Iguratimod (IGU), a disease-modifying antirheumatic drug launched in September 2012, has been reported to carry a risk of severe hemorrhages through a suspected interaction with warfarin (WF) in the all-case surveillance and early postmarketing-phase vigilance. To elucidate possible mechanisms of adverse interaction between IGU and WF, we analyzed the effects of IGU on the pharmacodynamics and pharmacokinetics of WF in rats. IGU was orally administered to male Wistar rats once daily for 5 d at 10 or 30 mg/kg in combination with WF at an oral dose of 0.25 mg/kg. Coadministration of IGU 30 mg/kg enhanced the anticoagulant activity of WF; prolonged blood coagulation time (prothrombin time and activated partial thromboplastin time) and decreased levels of vitamin K (VK)-dependent blood coagulation factors (II, VII, IX, and X) were observed. On the other hand, the pharmacokinetic parameters of WF including maximum plasma concentration (Cmax) and area under the plasma concentration-time curve from 0 to 24 h (AUC0-24 h) were not affected by the combination with IGU. IGU alone did not change blood coagulation time at doses up to 100 mg/kg, while VK-dependent blood coagulation factors decreased slightly at 30 and 100 mg/kg. These results suggest that the pharmacodynamic effect of IGU on VK-dependent blood coagulation factors is involved in the mechanism of drug-drug interaction of IGU with WF.