Half of them were children under the age of five, says the report by the UN food agency and the US-funded Famine Early Warning Systems Network.
The number of deaths was higher than the estimated 220,000 people who died during the 1992 famine.
The crisis was caused by a severe drought, worsened by conflict between rival groups fighting for power.
UN Food and Agriculture Organisation (FAO) senior economist Mark Smulders said the “true enormity of this human tragedy” had emerged for the first time from the study, done jointly with the Famine Early Warning Systems Network (Fews Net).
Western aid group ban
“By nature, estimating mortality in emergencies is an imprecise science, but given the quantity and quality of data that were available, we are confident in the strength of the study,” said Fews Net official Chris Hillbruner.
“It suggests that what occurred in Somalia was one of the worst famines in the last 25 years,” he added.
The UN first declared a famine in July 2011 in Somalia’s Southern Bakool and Lower Shabelle regions, which were controlled by the militant Islamist group al-Shabab, which is aligned to al-Qaeda.
It denied there was a famine and banned several Western aid agencies from operating in its areas.
The famine later spread to other areas, including Middle Shabelle, Afgoye and at camps for displaced people in the government-controlled capital, Mogadishu.
An estimated 4.6% of the total population and 10% of children under five died in southern and central Somalia, the report says.
“The report confirms we should have done more before the famine was declared,” said Philippe Lazzarini, UN humanitarian co-ordinator for Somalia.
“Warnings that began as far back as the drought in 2010 did not trigger sufficient early action,” he said in a statement, AFP news agency reports.
In Lower Shabelle, 18% of children under five died and in Mogadishu 17%, the report said.
Somalia was worst hit by extreme drought in 2011 that affected more than 13 million people across the Horn of Africa.
Tens of thousands of people fled their homes in search of food.
The UN declared the famine over in February 2012.
During more than 20 years of civil war, Somalia has seen clan-based warlords, rival politicians and Islamist militants battle for control – a situation that has allowed lawlessness to flourish.
Last September, a UN-backed government came to power, after eight years of transitional rule, bringing some stability to some areas.