The Nikkei 225 index at the Tokyo Stock Exchange opened 57.82 points down at 8,664.35.
S&P cited the EU's dependence on member states for its budget needs for the action. Earlier this week it issued a similar warning to nearly all of the eurozone countries.
Also on Wednesday it put a number of large European banks on review for a possible downgrade.
The euro eased to 1.3402 and 104.02 yen in early Asian trade from $1.3413 and 104.15 yen in New York late Wednesday.
The dollar was almost flat, changing hands at 77.65 yen.
Just before the Tokyo market opened, government data showed Japan's core private-sector machinery orders, a leading indicator of corporate capital spending, fell 6.9 percent in October from the previous month.
The government said in a separate report that Japan's current account surplus fell 62.4 percent from a year earlier in October, the eighth straight month of decline since the March earthquake and tsunami disasters.
US stocks erased losses in the last hour of trade Wednesday to end mostly higher with the Dow Jones Industrial Average closing 0.38 percent higher at 12,196.37.