Half of them were children under the age of five, says the report by the UN food agency and the Famine Early Warning Systems Network.
The number of deaths was higher than the estimated 220,000 people who died during the 1992 famine.
UN Food and Agriculture Organization (FAO) senior economist Mark Smulders said the "true enormity of this human tragedy" had emerged for the first time from the study.
"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.
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 percent of the total population and 10 percent 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-coordinator for Somalia.
"Warnings that began as far back as the drought in 2010 did not trigger sufficient early action," he said in a statement.
In Lower Shabelle, 18 percent of children under five died and in Mogadishu 17 percent, 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.