By 1250 GMT it traded at 155.94 per dollar, its biggest one-day gain in more than three years, after Finance Minister Satsuki Katayama and diplomat Atsushi Mimura signalled decisive action was near.
The move extended earlier gains from around 157 and 156.54, with traders citing possible official buying and short covering as Tokyo warned intervention could come on "all fronts," including energy markets.
The yen had slid past 160 amid oil-driven inflation fears linked to the Iran war; Japan last intervened in July 2024, and BOJ policymakers this week signalled rising concern over inflation.
Will Japan sacrifice its economy with higher rates just to save its struggling currency?
With its safe-haven status gone, is a full-blown yen crisis now inevitable?