"2018 is yet again an extremely warm year on top of a long-term global warming trend", Gavin Schmidt, director of GISS, said in a statement.
Last December was the second-warmest December in the 139 years that records have been kept.
To combat warming, nearly 200 governments adopted the Paris climate agreement in 2015 to phase out the use of fossil fuels and limit the rise in temperatures to 2C above pre-industrial times while "pursuing efforts" for 1.5 degrees Celsius.
Continuing a trend of rising temperatures, 2018 was the fourth hottest year on record, according to a climate change report out Wednesday from NASA and the National Oceanic and Atmospheric Administration (NOAA).
This year has also started with scorching temperatures, with destructive bushfires ravaging Tasmania as they endure their driest January ever.
Meanwhile, parts of the United States suffered bone-chilling cold from a blast of Arctic air last week.
In separate analyses of global temperatures, scientists from NASA, the United Kingdom Met Office and the World Meteorological Organizationoffsite link also reached the same heat ranking. Nine of the 10 warmest years have occurred since 2005, with the last five years comprising the five hottest.
"That is not saying the Paris Agreement is done for. but it's a worrying sign", he added.
NOAA said the average temperature for the contiguous U.S.in 2018 was 53.5 degrees Fahrenheit, making it a warmer-than-average year for the 22nd year in a row.
Scientists with NASA and NOAA will discuss the Earth's 2018 global temperatures and climate conditions on February 6, 2018. The Paris pact responded to a 1992 United Nations treaty under which all governments agreed to avert "dangerous" man-made climate change.
US President Donald Trump, who has cast doubt on mainstream climate science and promotes the coal industry, plans to withdraw the United States from the Paris Agreement.
Patrick Verkooijen, head of the Global Centre on Adaptation in the Netherlands, told Reuters that the WMO report showed "climate change is not a distant phenomenon but is here right now".
He called for more, greener investments, ranging from defenses against rising seas to drought-resistant crops.