UN agencies had estimated in June that 1.7 million of Sri Lanka’s 22 million residents were in need of assistance.
UN agencies in Colombo said in a joint statement they had raised $79 million to feed the needy, but the increasing number of poor meant they needed another $70 million.
Food insecurity in Sri Lanka has increased dramatically as a result of two consecutive seasons of poor harvests, foreign exchange shortages and reduced household purchasing power, the statement said.
Sri Lanka is facing its worst economic crisis since independence from Britain in 1948 and has been dealing with runaway inflation, power outages and fuel rationing since last year.
The country defaulted on its $51 billion foreign debt in mid-April and is in talks with the IMF for a $2.9 billion bailout.
Months of protests against high prices and shortages of food and medicine led to the fall of President Gotabaya Rajapaksa in July.
The UN said the revised plan aims to feed 2.1 million people, including pregnant mothers and schoolchildren, and provide livelihoods to 1.5 million farmers and fishermen.
It also said the poverty rate in the South Asian nation has doubled this year to 25.6 percent, from 13.1 percent last year.