Iran's currency, the rial, slumped 12 per cent in street trading on Monday, accelerating a slide triggered on the weekend when the United States activated new sanctions over Tehran's nuclear drive.
The rial was being exchanged for 17,800 to the dollar late Monday.
Before US President Barack Obama on Saturday signed into law the new sanctions targeting Iran's financial sector and central bank, the rial was trading around 15,500 to the dollar.
Over the past year, the rial has lost 66 percent of its value against the dollar.
The European Union is mulling its own additional sanctions, including a possible embargo on Iranian oil imports. If those are decided at an EU foreign ministers' meeting at the end of this month, Iran's rial could show further strain.
Iran has threatened to close the strategic Strait of Hormuz -- a passage at the entrance to the Gulf through which 20 percent of the world's oil flows -- if further sanctions are applied.
The United States and its allies have imposed the sanctions to pressure Iran to halt its nuclear programme, which they fear is being used to develop atomic weapons.
Tehran denies that, saying the programme is for exclusively peaceful purposes: generating energy and providing isotopes for medical use.