New Governor Haruhiko Kuroda committed the BOJ to open-ended asset buying and said the monetary base would nearly double to 270 trillion yen ($2.9 trillion) by the end of 2014, a dose of shock therapy officials hope will end two decades of stagnation.
The policy was viewed as a radical gamble to boost growth and lift inflation expectations and is unmatched in scope even by the U.S. Federal Reserve's own quantitative easing program.
The Fed may buy more debt, but since Japan's economy is about one-third the size of the economy, Kuroda's plan looks even bolder.
"This is an unprecedented degree of monetary easing," a smiling Kuroda told a news conference after his first policy meeting at the helm of the central bank.
"We took all available steps we can think of. I'm confident that all necessary measures to achieve 2 percent inflation in two years were taken today," he said.
One of those steps was to abandon interest rates as a target and become the only major central bank to target primarily the monetary base - the amount of cash it pumps out to the economy. It adopted a similar policy in 2001-2006, but not on this scale.