National records have fallen in eight countries - and regional records in another three.
BBC reports Warsaw, Poland, saw 18.9C (66F) on Sunday while Bilbao, Spain, was 25.1C - more than 10C above average.
The mild European weather comes as North America faces more severe storms, days after a deadly winter cold snap left more than 60 dead.
Heavy snow and freezing rain have been forecast for parts of the northern Midwest while severe thunderstorms and tornadoes are expected in Texas, Oklahoma, Arkansas and Louisiana.
But on the European side of the Atlantic, the weather has been balmy for many places at the start of the year.
Temperatures in the Netherlands, Liechtenstein, Lithuania, Latvia, Czech Republic, Poland, Denmark and Belarus broke national records.
Station records were broken in Germany, France and Ukraine.
The temperature recorded in Warsaw on 1 January was 4C higher than the previous record for the month, and Belarus' record high was 16.4C, some 4.5C above the previous record.
In Spain, New Year's Day temperatures in Bilbao were equivalent to the average in July, and parts of Catalonia including Barcelona are subject to restrictions on water use.