Not only was 2015 the warmest worldwide since 1880, it shattered the previous record held in 2014 by the widest margin ever observed, said the report by the National Oceanic and Atmospheric Administration.
"During 2015, the average temperature across global land and ocean surfaces was 1.62 Fahrenheit (0.90 Celsius) above the 20th century average," said the NOAA report.
"This was the highest among all years in the 1880-2015 record," it added.
The US space agency NASA, which monitors global climate using a fleet of satellites and weather stations, confirmed that last year broke records for heat in contemporary times.
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NASA said that the temperature changes are largely driven by increased carbon dioxide and other human-made emissions into the atmosphere.
"Climate change is the challenge of our generation," said NASA Administrator Charles Bolden.
"Today's announcement not only underscores how critical NASA's Earth observation programme is, it is a key data point that should make policy makers stand up and take notice -- now is the time to act on climate."
Moreover, the latest finding adds to a steady rise in heat across land and sea surfaces that have seen records repeatedly broken over the years.
"Since 1997, which at the time was the warmest year on record, 16 of the subsequent 18 years have been warmer than that year," said the NOAA report.
Last year alone, 10 months had record high temperatures for their respective months.
The heat was felt worldwide, with unprecedented warmth covering much of Central America and the northern half of South America.
Regions of eastern and southern Africa experienced more blistering heat than ever, as did large parts of the northeastern and equatorial Pacific boosted by the El Nino weather phenomenon.
A separate group, Berkeley Earth -- a US non-profit organisation that says it was founded by people who saw some merit in the claims of climate change skeptics -- announced similar findings last week.
"2015 was unambiguously the hottest year on record," it said in a statement.
NOAA's announcement comes against a backdrop of the recently completed Paris climate talks, at which the goal of capping global warming at 2 C above preindustrial levels was enshrined.