That helped lift the yen just a bit to 159.02 per U.S. dollar but the frail currency remains perilously close to 160, a level that has previously led Tokyo to intervene in the currency market. The yen has been rooted around 159 since mid-March.
The move from the BOJ may give some near-term support but ultimately the yen is unlikely to firm too much, analysts say. In short, markets can expect a continuation of the "carry trade", using the yen as a cheap funding currency.
In geopolitics, the U.S. was reviewing Tehran's latest proposal to resolve the war in the Middle East but a U.S. official said President Donald Trump was unhappy with the proposal because it did not address Iran's nuclear programme.
The uncertainty around the two-month-long conflict will surely keep oil prices supported, with Brent crude futures at $109 per barrel, well above the pre-war levels.
Investors' attention this week is on earnings from tech giants and central bank meetings this week.
The Federal Reserve, Bank of England and the European Central Bank are all due to announce policy decisions this week. All three are expected to hold rates unchanged but attention will be on what policymakers say about inflation and growth.
