Trump, meeting the pontiff for the first time, said he had "renewed" the US "commitment to fighting global famine", with the United Nations warning that about 20 million people across Nigeria, Somalia, South Sudan and Yemen are at risk.
The UN has described the situation as the biggest humanitarian crisis in its history, but donor funding is falling far short.
It appealed earlier this month for USD 4.4 billion, but said less than a third had been raised.
His announcement came just a day after the US State Department laid out plans to put "America first" and slash Washington's budget for diplomacy and foreign aid by more than 30 per cent.
The deepest cuts will hit foreign assistance programmes and contributions by the world's largest economy to international organisations like the United Nations and its global peacekeeping budget.