The won/dollar exchange rate finished at 1,350.4 won per dollar, up 19.1 won from the previous close.
It marked the highest close in over 13 years since April 2009 when the global financial crisis roiled financial markets across the world.
Powell said at the U.S. central bank's annual Jackson Hole symposium over the weekend that the Fed will raise rates "for some time" to counter inflation despite "some pain" that would be caused to households and businesses.
Powell's tough stance against inflation dented market hopes that the Fed may soon reverse course.
The Fed took a giant step for the second straight month in July to lift its benchmark interest rate by 0.75 percentage points to a range of 2.25-2.50 percent.
Powell left the door open for another "unusually large increase" at the Fed's next rate-setting meeting in September.
South Korea's foreign exchange authorities issued a verbal warning to slow the local currency's fast slide versus the greenback.
Bang Ki-sun, the country's first vice finance minister, said the government will make policy efforts for market stability in preparations for excessive herd behavior in the market.
Local stocks ended lower amid the domestic currency's weakness. The benchmark Kospi retreated 54.14 points, or 2.18 percent, to finish at 2,426.89, posting the lowest close in about a month.
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