Average world surface air temperatures were 14.7 Celsius (58.5 Fahrenheit) in 2018, 0.2C less than 2016 which was the hottest year on record, it said in the first global assessment of temperatures based on full-year data.
The year 2016 was boosted by an El Nino event that warmed the surface of the Pacific Ocean, Presstv Reported.
"In 2018, we have again seen a very warm year, the fourth warmest on record," Jean-Noël Thépaut, head of the Copernicus Climate Change Service, said in a statement.
"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," he added.
The years 2017 and 2015 were also fractionally warmer than 2018 in records dating back to the 19th century, Copernicus said.
Almost 200 nations agreed to a "rule book" to govern the Paris climate agreement in talks in Poland last month, even though critics said it was insufficient to prevent dangerous levels of global warming.
The 2015 Paris accord seeks to end the fossil fuel era this century by shifting to cleaner energies such as wind and solar power.
US President Donald Trump plans to pull out and instead promote the US fossil fuel industry.
The Copernicus report confirms projections by the UN’s World Meteorological Organization (WMO) in November that 2018 would be fourth warmest, after a cool start to the year.