The United Nations says 12 million people in Ethiopia, 5.6 million in Somalia and 4.3 million in Kenya are "acutely food insecure".
The overall figure has almost doubled from 13 million at the start of 2022, the UN's World Food Programme said in a January 23 report.
Across the region, 1.7 million people have been driven from their homes by a lack of water and pasture, it said.
Somalia is the hardest hit country, with the drought affecting more than half of its population, about 7.85 million people.
In December, the UN's humanitarian agency, OCHA said the troubled nation was technically not yet in the grip of full-blown famine thanks to the response of aid agencies and local communities.
But people were nevertheless suffering "catastrophic" food shortages, it said, warning that if assistance is not scaled up, famine is expected in southern Somalia between April and June.
Agro-pastoral populations in Baidoa and Burhakaba districts, and displaced people in Baidoa town and the capital Mogadishu were most at risk.'
Crops, already ravaged by a locust invasion between 2019 and 2021, have been wiped out and livestock has suffered a similar fate.
OCHA estimated in November that 9.5 million cattle had perished.
Humanitarian groups warn that the situation is only likely to worsen, with the next rainy season from March to May also expected to be below average.
Dire conditions in the Horn have been amplified by the war in Ukraine, which has contributed to soaring food and fuel costs, disrupted global supply chains and diverted aid money away from the region.
According to the UN children's agency UNICEF, almost two million children across Ethiopia, Kenya and Somalia require urgent treatment for severe acute malnutrition, the deadliest form of hunger.
It said in September that 730 children had died between January and July in nutrition centers in Somalia, but that the real numbers were likely much higher.
Lacking water, milk and food, and often living in squalid conditions, the youngest become so weak they are vulnerable to diseases such as measles and cholera, and their long-term growth can be restricted.
Some 2.7 million children have also stopped going to school, it said.
"There is no end in sight for the hunger crisis and hope is slowly fizzling out," said Xavier Joubert, Ethiopia director for the British charity Save the Children.
"There's no doubt that the need has grown to an enormous scale," he said, adding that more funds were urgently required.