The grim warning is contained in a research report by the University of Chicago-based Climate Impact Lab released on Wednesday.
As many as 451,000 more heat-related deaths could occur between June and the end of February that could be attributed to El Nino, compared with heat deaths expected in a typical year.
The heaviest toll, the group says, is expected in poorer countries, already struggling with food insecurity and high energy costs, partly due to climate change and the fallout of the war in Iran.
Michael Greenstone, a co-founder of the group and the study's co-author, said the heat danger will likely persist beyond February - even though this El Nino is expected to end by then. The fallout of the weather phenomenon could be felt well into next summer, he added.
The research, he said, shows that heat is most dangerous in places that are already hot and relatively poor.
"If you're already in a hot place, the extra couple of degrees are going to be much more dangerous," he said.
Wealthier places, he added, generally have greater access to air conditioning, reliable water supplies and functioning health systems.
Researchers estimated the additional heat-related deaths by comparing deaths expected under El Nino-driven high temperatures with deaths expected if temperatures remained similar to what they have been in the last three decades.
The projected fatalities are for the entire El Nino period, from June 2026 until the end of February 2027.
The study also warns that the Global South - a term that isn't geographical but a reference to developing countries in general - will bear the heaviest burden of the excessive heat.
The Sahel region in Africa - countries along the semiarid southern fringe of the Sahara Desert - is expected to be the hardest-hit, with about 66,800 additional deaths caused by El Nino-driven heat.
Other major hot spots include South and Southeast Asia and parts of Latin America.
The researchers project about 19,400 additional heat-related deaths across the Philippines, Vietnam, Thailand and Cambodia, as well as 19,300 in Indonesia, 15,800 in India and 13,300 in Brazil.
Recommended measures that governments can take include: heat-warning systems, cooling centres, hydration stations, protections for outdoor workers, expanded hospital capacity and targeted outreach to elderly people and other vulnerable groups.
The authors also call for what they describe as anticipatory financing, so governments can move resources before dangerous heat peaks.
El Nino is a natural climate pattern, but this year's unusually strong event is occurring on a planet already warmed by human-caused climate change, increasing the risks from extreme heat.
The Climate Impact Lab said global land temperatures during this El Nino could be higher by about 1.2C, producing 44 per cent more extremely hot days than in a typical year.
