Temperatures climbed to 10.4C in central Swedish towns Eskilstuna and Adelsö on the last day of the year, said national weather institute SMHI, looking at its digital records between 1951 and 2016.
That breaks Sweden's previous record, set in Duved in 1972, by 0.1 degrees.
Stockholm meanwhile had its warmest New Year's Eve in at least 157 years, according to the weather agency, whose station in the capital has been keeping records of hottest temperatures since 1859.
In 1975 8.4C was measured in Stockholm, compared to 10.3C this December 31st.
“The old record is broken by a wide margin,” meteorologist Lisa Frost told the TT newswire.
SMHI measured temperatures between 7pm on January 30th and 7pm on January 31st.
Stockholm in particular had a real record-breaking winter in 2016. In November it even grabbed international headlines after having its snowiest November day in 111 years.
And anyone hoping temperatures will stay warm in 2017 are not likely to have their wish granted.
“This is the last sigh of the real heat. On Monday it's going to be below freezing in the whole country,” she said.