Over September, October and November, the mean temperature in the UK was 10.87C (51.57F), and in Northern Ireland the average was 10.95C (51.71F), forecasters said.
England had its fourth warmest autumn since records began in 1884, with a mean temperature of 11.64C (52.95F).
Meanwhile, Scotland experienced the third warmest with an average temperature of 9.48C (49.06F), while Wales also saw the third warmest at 11.18C (52.12F).
Dr Mark McCarthy, of the National Climate Information Centre, described November as a markedly “mild and dry month” for all areas except the far north west.
He said: “Although many will remember November for the impacts of Storm Arwen, particularly in the North East, the month as a whole has been fairly dry in many areas, with slightly above average temperatures for the period as well.
“A mild and dry month are the main takeaways from November 2021, with the exception of the far north west, which had above average rainfall.”
For Northern Ireland, the unusually warm autumn followed the nation’s third warmest summer on record, during which a new maximum temperature was set.
The record was broken when the mercury hit 31.3C (88.34F) at Castlederg in Co Tyrone during July.
September was also the second warmest on record for the UK, and was followed by a warm and wet October which saw mean temperatures rise 1.4C above average.
Most of the UK experienced a slightly drier autumn than usual, with 93% of the average rain falling across the period.
The data confirms what many in Northern Ireland were already reporting with flowers continuing to bloom into November and wasps still being seen.
However, some northern areas of the UK did experience wetter weather, including Orkney where more than a third more of its average rain was recorded at 480.6mm.