The common currency bought $1.2784 in Tokyo afternoon trade from $1.2811 in New York late Friday, while it slipped to 100.00 yen from 100.21 yen.
The dollar was quoted at 78.22 yen, slightly up from 78.20 yen in US trade.
On Friday, the euro jumped above 100 yen for the first time in two months after the ECB said it would buy bonds from troubled eurozone nations in a bid to bring down their heavy borrowing costs and prop up the currency.
The unit had also gained on the dollar Friday after US data showed the economy added only 96,000 jobs in August, well below the 130,000 estimated.
"The EUR/USD may have climbed too high, and caution is leading to position adjustments," said Masashi Murata, senior currency strategist at Brown Brothers Harriman in Tokyo.
Euro sentiment was dampened by broader worries about the bloc sinking into recession, while traders were looking to Wednesday's German court ruling on the European Stability Mechanism bailout fund (ESM), dealers said.
The court, which was set to rule on the legality of the fund, was not expected to block the ESM.
But any surprise decision could mean the eurozone finds itself short of funds to prop up indebted members such as Spain and Italy.
"In truth, moves on the week are much more likely to be governed by international developments, specifically from 'Super Wednesday' when the German Constitutional Court will rule of the legality of the expanded powers of the ESM," National Australia Bank said in a note.
The US Federal Reserve, meanwhile, concludes a policy meeting on Thursday with markets on high alert for any signs that the central bank will launch further stimulus.
The dollar was lower against other Asia-Pacific currencies.
It fell to Sg$1.2353 from Sg$1.2429 on Friday, to 1,128.85 South Korean won from 1,130.05 won and to 41.60 Philippine pesos from 41.75 pesos.
The greenback slipped to 31.07 Thai baht from 31.25 baht, to 55.31 Indian rupees from 55.61 rupees, to Tw$29.67 from Tw$29.80 and to 9,578 Indonesian rupiah from 9,592 rupiah.
China's yuan bought 12.34 yen against 12.44 yen, while the Australian dollar was at $1.0366 from $1.0314.