Last month was the sixth warmest January on record, the Met Office has said.
The average mean temperature across the UK was 5.6C, according to provisional figures.
The warmest January on record was in 1916, when the mean temperature was 6.3C.
Mild conditions meant the number of days of air frost last month was well below average.
Air frost is defined by the Met Office as a day when the minimum temperature falls below freezing (0C).
Only five days of widespread air frost were recorded in January 2020 – the lowest number since 1990.
In Morpeth, Northumberland, a 135-year January record was overturned when the local weather station did not observe a temperature of 0C or below during the entire month.
Met Office data on temperatures in the UK begins in 1884.
Figures for average rainfall in January 2020 show that Northern Ireland received only 59% of its long-term average, while England and Wales received 86% and 91% respectively.
Only Scotland (119%) received an above average amount.
The brightest parts of the country were along the east coast, with Leconfield in the East Riding of Yorkshire taking the prize for the sunniest place (145% of average sunshine for January).
By contrast, both Stornoway and Tiree in the Hebrides recorded their dullest January on record, with just 42% and 38% of average sunshine respectively.