Last year was the fourth warmest on record, extending a scorching streak driven by a build-up of man-made greenhouse gases.
Average world surface air temperatures were 14.7 Celsius in 2018, just 0.2C off the highest, the European Union's Copernicus Climate Change Service said in the first global assessment based on full-year data.
This year will also likely be hot, its scientists said.
"Dramatic climatic events like the warm and dry summer in large parts of Europe or the increasing temperature around the Arctic regions are alarming signs to all of us," said Jean-Noel Thepaut, head of Copernicus.
Among other extremes in 2018, California and Greece suffered severe wildfires, Kerala in India had the worst flooding since the 1920s and heatwaves struck from Australia to North Africa.
Around Antarctica, the extent of sea ice is at a record low at the start of 2019, according to the US National Snow and Ice Data Center.
The last four years have seen the highest average temperatures since records began in the 19th century - 2016 was the hottest, boosted by an El Nino event that warmed the surface of the Pacific Ocean, ahead of 2017 and then 2015.
The Copernicus report said that concentrations of heat-trapping carbon dioxide in the atmosphere rose to a new record of 406.7 parts per million (ppm) in 2018 from 404.1 in 2017, stoked largely by human burning of fossil fuels.
And the average global temperature in the past five years was 1.1C above pre-industrial times, it said.
According to a UN climate report last year, temperatures will rise 1.5C above pre-industrial times by mid-century on current trends - bringing the prospect of even more extreme weather.
The Copernicus report confirms projections by the UN's World Meteorological Organisation (WMO) in November that 2018 would be fourth warmest.
Australian Associated Press