In afternoon Tokyo trade, the greenback bought 83.59 yen, its highest level against the Japanese currency since late March, and up from 83.24 yen in New York Wednesday afternoon.
The euro soared to 109.40 yen, its highest since early April, from 108.85 yen in US trade, while the 17-nation currency also firmed to $1.3084 from $1.3075.
The safe-haven yen has weakened as the frontrunner to become Japan's next prime minister repeatedly said he would pressure the Bank of Japan (BoJ) for more aggressive easing to prop up the world's third-largest economy.
Shinzo Abe, head of the main opposition Liberal Democratic Party, has called on policymakers to take stronger action, with official data earlier this week signalling that Japan was slipping into recession.
After a two-day meeting, the policy committee of the US central bank said it would replace its "Operation Twist" bond swapping programme with $45 billion a month in straight bond buys, on an open-ended basis.
That comes on top of the $40 billion a month purchasing announced earlier.
The Federal Reserve also provided a surprise by saying it would not lift rates as long as the inflation outlook was below 2.5 percent and the jobless rate, now at 7.7 percent, stays above 6.5 percent.
The Fed move has stoked expectations of further easing by the European Central Bank and Japanese policymakers, dealers said.
"Globally, we expect further easing by central banks, and stocks are rising in turn. The yen as a result should weaken," a senior bank dealer told Dow Jones Newswires.
The dollar was mixed against other Asia-Pacific currencies on Thursday.
It slipped to 1,072.13 South Korean won from 1,075.30 won on Wednesday, to 54.16 Indian rupees from 54.24 rupees, to 9,699 Indonesia rupiah from 9,700 rupiah and to Tw$29.05 from Tw$29.07.
The greenback edged up to 30.63 Thai baht from 30.60 baht and to 41.02 Philippine pesos from 40.99 pesos, while it was flat at Sg$1.2212.
The Australian dollar firmed to US$1.0548 from US$1.0526 while China's yuan bought 13.31 yen from 13.22 yen.