Leaders of the G8 nations say international development banks will give $20 billion to Egypt and Tunisia to support pro-democratic reforms.
President Barack Obama and other leaders meeting in France said in a final statement that their countries will also "mobilise substantial bilateral support to scale-up this effort".
The statement did not provide breakdowns of what aid each G8 country would provide.
The leaders encouraged other countries, including rich Arab world nations, to contribute as well to shoring up economies in Egypt and Tunisia, where Arab Spring uprisings this year overthrew dictators but also scared away tourists and investors.
The G8 statement said: "In the short term, our collective aim is to ensure that instability does not undermine the process of political reform, and that social cohesion and macroeconomic stability are both sustained."
The billions in aid from multilateral development banks is aimed at "suitable reform efforts" from now through to 2013, the statement said, without elaborating. About 3.5 billion dollars would come from the European Investment Bank.
The leaders did not lay out exactly what the money would be for, or whether the figure includes money already promised for the region. US and European officials had said it was too early to come up with firm figures.
After meetings with the leaders of Egypt and Tunisia in the French resort of Deauville, the G8 leaders launched a partnership programme aimed at supporting the countries' fragile political leadership, fighting corruption and stabilising economies.
The G8 leaders laid out a plan for refocusing the European Bank for Reconstruction and Development - created to help eastern European economies after the collapse of communism - to help Arab democracies.
The EBRD was set up 20 years ago when the collapse of the Soviet Union convinced European leaders of the urgency to provide support to a region emerging from decades of political and economic dictatorship.
The idea was to set up a "transition bank" to help lead the way on banking systems reform, price liberalisation, privatisation and establishing legal property rights in a region shaking off the effects of almost 50 years of planned economies.
The G8 leaders are also worried that fighting in Libya and violence against protesters in Syria could derail the pro-democracy movement that has swept around the Arab world since Tunisian protesters rose up against an autocratic regime and forced out their long-time president.
In their final statement, the G8 leaders said Libyan leader Muammar Gaddafi "must go" and are pressing Syria's regime to "stop using force and intimidation" against its people.
The G8 leaders say Gaddafi and his government have failed to fulfil their responsibility to protect Libya's people "and have lost all legitimacy. He has no future in a free, democratic Libya".