The Japanese currency jumped as much as 2.6 per cent against the dollar in the wake of the shock announcement, with the greenback plunging to 108.58 yen in afternoon trade.
Markets had widely expected new measures to counter a downturn in the world's number three-economy.
That speculation had picked up pace after Japan published a slate of weak data -- including the biggest drop in inflation for three years -- shortly before markets opened Thursday.
"It's a total shock," Nader Naeimi, the Sydney-based head of dynamic markets at AMP Capital Investors, told Bloomberg News.
"From currencies to equities to everything -- you can see the reaction in the markets. I can't believe this. It's very disappointing," Naeimi added.
By the close on Thursday, Tokyo's Nikkei 225 index had dived 3.61 per cent, or 624.44 points, to 16,666.05, its fourth straight negative finish. It lost 5.16 per cent over the week.
The broader Topix index of all first section shares slid 3.16 per cent, or 43.75 points, to 1,340.55. The index was down 4.76 per cent over the week.
Nintendo plummeted 8.26 per cent to 15,155 yen following its announcement after markets closed Wednesday that fiscal year net profit dropped 60 per cent from a year ago, while sales also fell.
Industrial robot maker Fanuc and leading brokerage firm Nomura also nosedived, respectively losing 9.93 per cent to end at 16,505 yen and 10.04 per cent to 478.9 yen, as disappointing financial results drove their shares down.
Exporters slipped on the stronger yen. Toyota was down 3.26 per cent to 5,654 yen, Nissan tumbled 4.30 per cent to 1,000 yen, and Sony lost 2.07 per cent to 2,778 yen.
Banking giant Mitsubishi UFJ shed 5.94 per cent to end at 520.1 yen, while rival Sumitomo Mitsui Financial Group was off 6.00 per cent at 3,413 yen.
Some firms, however, still carved out gains on upbeat financial results as Japan's earnings season revs into high gear.
Among the major movers, Apple-supplier Alps Electric skyrocketed 9.94 per cent to 1,990 yen on upbeat earnings results, while crisis-hit Mitsubishi Motors rebounded 6.39 per cent to 449 yen after a hair-raising tumble since last week.
Investors had lopped about 50 per cent off Mitsubishi's shares since the embattled automaker admitted a week ago it has been cheating on fuel-efficiency tests.
Tokyo equity markets will be closed on Friday and from Tuesday to Thursday next week for public holidays.
- AFP