The dollar was last trading up 0.1 percent at 99.84 yen, after having risen as high as 99.94, its highest level since April 2009.
Technical resistance is seen at 99.73 yen—the 50 percent retracement of the dollar's drop from its June 2007 high of 124.14 yen to a record low of 75.311 yen set in October 2011.
Traders cited hefty offers to sell dollars at 100 yen from Japanese exporters. They added, however, that most investors were looking to use dips to add to long-dollar positions.
A rise above 100 yen would open the door for a test of the April 2009 peak of 101.45 yen, analysts said.
Ulrich Leuchtmann, head of currency research at Commerzbank in London, said it was possible that the dollar could rise to 115 yen or higher by year-end, but added it was difficult to predict the extent of the move.
"The 100-yen level is a psychological level that might take a bit of time to break, but this is still a very significant qualitative change by the Bank of Japan that is not fully priced in yet," said Leuchtmann.
Japan's aggressive monetary easing contrasts with expectations the Federal Reserve may slow its bond buying later this year. These expectations were given a boost as minutes from a recent Fed meeting released on Wednesday showed a few U.S. policymakers looking to taper asset purchases by mid-year.
Euro Hits 3-Year High
The euro rose to its highest against the yen in more than three years, hitting 131.13 yen on Reuters data, and was last trading up 0.3 percent on the day at 130.78.
Against the dollar, the euro rose 0.2 percent to $1.3098, after a session peak of $1.3138, the highest since the end of February.
Speculation was growing that the BOJ's ultra-loose policy would drive Japanese investors to riskier and higher-yielding foreign assets, with the euro one of the beneficiaries.
So far, however, there was little evidence of that happening. Data from Japan's Ministry of Finance showed Japanese investors sold a net 1.145 trillion yen ($11.5 billion) worth of foreign bonds last week, the highest sales in a year.
The yen was particularly softer against higher-yielding currencies, such as Australian and New Zealand dollars, as investors borrow in yen cheaply and use the proceeds to invest in such higher-yielding currencies in what are known as carry trades.
The Australian dollar rose to a five-and-a-half year high of 105.43 yen while the New Zealand dollar hit 86.43 yen, its strongest since early 2008.