They say 40% of coral has died at the Dongsha Atoll in the South China Sea.
Nothing as severe has happened on Dongsha for at least 40 years, according to experts.
Anne Cohen of Woods Hole Oceanographic Institution in Massachusetts, US, said the high water temperatures of 2015/16 were devastating for reef systems globally, including Dongsha.
Coral bleaching - where corals turn white and may die - was the worst on record for Australia's iconic Great Barrier Reef in 2016.
The barrier reef has absorbed a lot of the attention, but other reefs around the world were also severely affected, said Dr Cohen.
"The 2015/2016 El Nino was devastating for reef systems in other parts of the world as well, including Dongsha Atoll and reefs in the central Pacific, where some of the most pristine coral reefs are located and of course, the US Pacific Remote Marine National Monument," she said. "We observed devastating bleaching in that area as well."
Only last week, scientists published observations of three major die-offs of coral at the Great Barrier Reef in 2016, 2002 and 1998.
They concluded that the only way to preserve the world's coral reefs is to take drastic action to reduce global warming.
The study of the Dongsha Atoll, reported in the journal, Scientific Reports, echoes this finding.
"Based on what we observed on Dongsha, a 2 degree cap on ocean warming may not be enough to save coral reefs," Dr Cohen told BBC News.
"This is because coral reefs are shallow water ecosystems and a tweak in the local weather can turn that 2 degrees Celsius into a 6 degrees Celsius warming."
The Dongsha Atoll, located in the South China Sea, near south-eastern China and the Philippines, is rich in marine life and is regarded as one of the world's most important coral reefs.
The researchers said on its own, a 2 degrees Celsius rise in temperatures was unlikely to cause widespread damage to coral reefs in the region.
But, a high-pressure system caused temperatures to spike to 6 degrees, leading to the death of 40% of coral over the course of six weeks.
They argue that predictions of the future of coral reefs may be "overly optimistic" for some reefs in shallow water.
Bleaching happens when high water temperatures cause corals to expel the algae they depend upon.
The Australian government confirmed in March that widespread coral bleaching is happening on the Great Barrier Reef for the fourth time in history.
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Scientists are warning of another "devastating" loss of coral due to a spike in sea temperatures.